Go in-depth with inbound and outbound SIP trunking, and understand which features make the most sense for your business needs.
Go in-depth with inbound and outbound SIP trunking, and understand which features make the most sense for your business needs.
There are a lot of ways to classify calls. And there are reasons for classifying calls according to their type. But classifying calls by which direction they travel is one of the most useful ways to categorize your calls.
That's why understanding inbound SIP trunks vs outbound is valuable, and that's what this article will help you do.
In short, there are two major categories of calls: inbound and outbound. When someone calls your personal or business phone number, that's an inbound call. If the call goes the other direction, you call someone else, that's an outbound call. Typically, your inbound and outbound calls connect through the same SIP trunk.
However, it's possible to have one SIP trunk for your inbound calls and another SIP trunk for your outbound calls. Or you can have a SIP trunk for calls that go in one direction, and a PRI trunk for calls that go in the other direction.
This may sound odd. But there are reasons to have separate trunks (either a ) for your inbound and outbound calls.
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"SIP is a technology that can and should coexist with cloud telephony for one simple reason: enterprises still need to connect to the PSTN. Platforms such as Microsoft Teams or Cisco WebEx offer powerful new communication and collaboration tools, but on their own, they cannot encompass every possible voice calling scenario."
— James McCall, Director of Voice Product Management, GTT Communications
The SIP trunking market continues to expand rapidly as enterprises transition away from legacy phone systems. According to Mordor Intelligence, the global SIP trunking market reached USD 73.14 billion in 2025 and is projected to reach USD 157.91 billion by 2030, growing at a 16.64% CAGR.
Key market insights include:
Inbound SIP trunking is using a separate SIP trunk for inbound calls, while using a separate SIP trunk or PRI trunk for your outbound calls. This enables you to pay for only the inbound calling features you need, while keeping your outbound calling more affordable.
Inbound and outbound calls have different capabilities, and therefore different feature sets. If your business only needs the features of inbound calling, you can get a fully featured inbound SIP trunk for your inbound calls. Then you can pair this with a relatively stripped-down outbound trunk to avoid paying for outbound call features that you don't use.
Whether or not this is a good option for your business will depend on how you use your telephone infrastructure. For businesses building sophisticated customer engagement solutions, combining inbound SIP capabilities with contact center AI can significantly enhance caller experiences through intelligent routing and automated responses.
Most inbound SIP features improve the caller experience. They're designed to save customers from busy signals or reaching the wrong person or department, and make it as easy as possible to call you.
These are the features you can add to your inbound SIP trunk:
Toll-free phone numbers: Toll-free phone numbers make it free for customers to call you. This is ideal for sales and customer service lines, where making the customer pay for the call is bad practice.
Custom phone numbers: Custom phone numbers, especially numbers that spell out a word based on the letters on the keypad, make it easy for customers to remember your business phone number. Custom phone numbers are also great if you need to use a non-fixed VoIP number to make it look like your phone number is local to the caller's geographic area.
Direct voicemail delivery: If you'd rather not put customers on hold when all your customer service representatives are busy, voicemail is your best option. This will deliver voicemail messages directly to your employees' inboxes, rather than a central business inbox.
Voicemail transcription: No matter how you need your voicemail messages delivered, voicemail transcription can make it much easier to retrieve and review. With voicemail transcription, voicemails can be automatically transcribed and delivered via chat or text message.
Custom call routing: In an office or organization with many phones and phone numbers, custom call routing is vital for connecting customers to the right people. For businesses looking to maximize the value of inbound calls, implementing IVR systems for call centers can reduce costs while keeping callers satisfied through intelligent routing.
Call recording: There are multiple reasons to record calls. The most common reason is training. But, in some industries, call recording may be required for legal compliance.
For detailed configuration options for inbound call handling, see the SIP Connection inbound and outbound settings documentation.
If your business requires any or all of these features, an inbound SIP trunk is the best solution. PRI trunks simply aren't capable of delivering all of these features.
Outbound SIP trunking is the opposite of inbound SIP trunking: placing your outbound calls on a separate SIP trunk from your inbound calls. Your outbound SIP trunk enables your business to reach out to customers from multiple phone numbers and integrate your outbound call infrastructure with your data management software.
Using a separate outbound SIP trunk is ideal for businesses that make a lot of calls while fielding fewer inbound calls. Think telemarketing call centers or census outreach contractors.
An outbound SIP trunk gives you all the outbound phone numbers and features you need, while maintaining just enough ability to accept calls. But, just like inbound SIP trunking, whether or not you should use a separate inbound SIP trunk depends on your ratio of inbound to outbound calls, and what capabilities you need for making calls.
The main idea of outbound SIP features is making it easier for you to call the right phone numbers and manage all the data associated with your phone numbers.
An outbound SIP trunk can deliver these features:
CRM and data management software integration: If you make high volumes of outbound calls, you most likely have a lot of data attached to the phone numbers you call. It's best if your phone number data is integrated into your calling infrastructure, so employees can access the information on-demand.
Call data capture: Detailed call records are valuable for analyzing your outbound calling and optimizing call flows, maximizing completed calls, and evaluating employee performance. Capturing and maintaining call data may also be required for regulatory compliance, depending on your industry.
Caller ID: Caller ID is essential for any business that makes calls. If people don't know who's calling them, they will usually ignore the call. It's important that your business is correctly identified, regardless of which device or phone number you're calling from.
Emergency calling: Making emergency calls through a SIP connection isn't quite the same as making an emergency call on a traditional telephone network. But a quality outbound SIP trunk provider will be able to connect emergency calls on your outbound SIP trunk.
When evaluating SIP trunking options, consider how different providers stack up across key criteria:
| Feature | Traditional PRI | Basic SIP Trunk | Elastic SIP (Telnyx) |
|---|---|---|---|
| Scalability | Fixed channels (23 per PRI) | Manual provisioning | Instant, unlimited scaling |
| Pricing model | Monthly per-channel fee | Per-channel or per-minute | Pay-as-you-go |
| Setup time | 2-4 weeks | 1-2 weeks | Minutes |
| Geographic flexibility | Local only | Limited regions | 140+ countries |
| Failover/redundancy | Additional hardware required | Provider-dependent | Built-in automatic failover |
| API access | None | Limited | Full REST API |
| Number porting | Manual, lengthy process | Provider-assisted | Self-service portal |
| HD voice support | No | Provider-dependent | G.722, Opus, AMR-WB |
| MS Teams integration | Requires gateway | BYOC options | Native Direct Routing |
| Typical cost savings vs PRI | Baseline | 25-40% | 50-65% |
Understanding the technical requirements ensures your SIP trunk deployment runs smoothly. Here are the key specifications for Telnyx SIP trunking:
| Specification | Details |
|---|---|
| Supported audio codecs | G.711 (μ-law/A-law), G.729, G.722, Opus, AMR-WB |
| SIP signaling ports | UDP/TCP: 5060; TLS: 5061 |
| RTP media ports | 1024-65535 (dynamically negotiated) |
| Transport protocols | UDP, TCP, TLS 1.2+ |
| Media encryption | SRTP (optional, configurable) |
| DTMF method | RFC 2833 (recommended), SIP INFO |
| SIP domains | sip.telnyx.com (US), sip.telnyx.com.au, sip.telnyx.eu, sip.telnyx.ca |
| Codec | Bandwidth per call | MOS score | Best use case |
|---|---|---|---|
| G.711 | ~87 kbps (with overhead) | 4.1-4.4 | High-quality audio, sufficient bandwidth |
| G.729 | ~31 kbps (with overhead) | 3.8-4.0 | Bandwidth-constrained environments |
| G.722 | 48-64 kbps | 4.5+ | HD voice applications |
| Opus | 32-96 kbps (adaptive) | 4.5+ | Variable network conditions |
For optimal call quality, maintain latency below 150ms one-way, jitter under 30ms, and packet loss below 1%. More details on codec selection and configuration are available in TechTarget's SIP bandwidth guide.
| SLA metric | Telnyx commitment |
|---|---|
| Network uptime | 99.999% availability |
| Voice quality (MOS) | ≥4.0 target |
| Support availability | 24/7/365 |
| Number porting SLA | 7-14 business days (US) |
| New number provisioning | Instant (API/Portal) |
If you recall from the beginning of the article, it's possible to have your inbound SIP trunking and outbound SIP trunking on the same trunk. And this is usually the most cost-efficient and reliable solution.
Even if you only use SIP trunking for calls in one direction, with a PRI trunk for calls in the other direction while you make the transition from traditional phone lines to SIP trunking, you'll get the best pricing and reliability if all your SIP trunking comes from the same provider.
For enterprises looking to modernize their entire communications stack, integrating SIP trunking with a UCaaS platform provides a unified solution for voice, video, and messaging.
Ready to configure your SIP trunk? These resources will help you get started:
That's where Telnyx comes in. Telnyx offers both outbound and inbound SIP trunks. And the Telnyx network is capable of supporting all the features you need, regardless of your calling requirements. Talk to a Telnyx expert to find out how you can get everything you need from a single SIP trunking provider.
Have questions about inbound SIP trunks? Join r/Telnyx on Reddit. https://www.reddit.com/r/Telnyx/
