Porting to SIP Trunks: DID Configuration and Routing in Your PBX

Switching from old phone lines to SIP trunks isn’t just about upgrading hardware-it’s about rethinking how your business talks. If you’re moving away from PRI lines or analog circuits, you’re not just saving money-you’re gaining flexibility, scalability, and control. But the real challenge? Getting your DID numbers to work right inside your PBX. A single misconfigured digit can send customer calls to voicemail, route international calls to the wrong department, or worse-open the door to fraud.

Why SIP Trunks Replace Traditional Lines

SIP trunking connects your PBX to the outside world using the internet instead of physical phone lines. It’s not VoIP in the old sense-this is enterprise-grade telephony built on standardized protocols. Since the IETF published RFC 3261 in 2002, SIP has become the backbone of modern business communications. Today, 68% of mid-to-large businesses in North America have already made the switch, according to Nemertes Research’s 2024 report. Why? Because it cuts costs by 30-50% compared to PRI lines and lets you add or remove phone numbers in minutes, not weeks.

But here’s the catch: SIP trunks don’t work unless your DID numbers are properly mapped. A DID (Direct Inward Dialing) number is the public phone number people dial to reach you. Each one needs to be told exactly where to go inside your PBX-whether it’s to a person, a call queue, or an IVR menu. If that mapping fails, your customers hear silence or a wrong number.

The Core Workflow: Step-by-Step

No matter if you’re using 3CX, FreePBX, or Yeastar, the steps are the same. Skip one, and your system won’t work.

1. Buy your DIDs through your SIP provider’s portal (like DID Logic or Bandwidth). Make sure you select the right country and number format-E.164 is standard (e.g., +15551234567).
2. Configure the SIP trunk in your PBX. You’ll need the provider’s SIP server address, your username, password, and registration port (usually 5060). Some systems, like 3CX, ask you to pick ‘Generic SIP Trunk’; others, like Yeastar, have preset templates for the U.S.
3. Set your outbound caller ID. This is often the same as your main DID. If you don’t set this, callers might see a random number or your provider’s default.
4. Create inbound rules. This is where most failures happen. Each DID must have a rule that says: “When this number is called, route to [extension/queue/IVR].” Be exact. If your DID is +15551234567, your rule must match that format-not 5551234567, not 15551234567 without the +.
5. Set up outbound routes. Define which numbers can be dialed and which trunk to use. For example, all 10-digit U.S. calls go out via SIP trunk, international calls go through a different route with a prefix like 011.

Many admins miss step four. A Reddit thread from June 2024 had 147 comments from people stuck because their DID rule used “1NXXNXXXXXX” instead of “+1NXXNXXXXXX.” The system saw it as a mismatch and dumped the call to the default route. Always match the E.164 format your provider sends.

Platform Differences Matter

You can’t treat all PBX systems the same. Here’s how three popular platforms handle it:

Comparison of DID Configuration Across Major PBX Platforms

Platform	Trunk Setup	DID Inbound Rule Method	Common Pitfall
3CX (v18.0.7)	Generic SIP Trunk, regional proxy hostname	Visual drag-and-drop rules; auto-detects DID format	Forgetting to enable “Use this trunk for outbound calls”
FreePBX (v16)	Add SIP (chan_pjsip) trunk; manual SIP settings	Manual dial pattern rules like “Prepend: 1 | Match: NXXNXXXXXX”	Incorrect regex patterns causing calls to bypass rules
Yeastar S-Series	ITSP template → Select “United States” + “SIPTRUNK”	Assign DID directly to extension via dropdown	Leaving Caller ID Number field blank

3CX scores highest for ease of use-Gartner gave it 4.6/5 for SIP setup. FreePBX is powerful but demands technical skill. If you’re not comfortable with regex or CLI commands, 3CX or Yeastar will save you hours.

Security and Fraud Risks

A poorly configured DID can turn your system into a fraud magnet. In 2023, SIP trunk fraud increased 300% globally, according to Fraudmarc’s report. One common scam? Hackers find a misconfigured DID that allows outbound international calls, then make thousands of dollars in calls to high-cost countries. Your provider bills you.

Here’s how to lock it down:

• Enable TLS 1.2+ and SRTP encryption on your SIP trunk.
• Set call limits per DID-DID Logic defaults to 2 simultaneous calls. Raise it only after testing.
• Block international calls unless explicitly needed. Use outbound route restrictions.
• Turn on real-time fraud monitoring if your provider offers it (DID Logic blocks suspicious calls in under 200ms).

Dr. Steve Taylor from Packetizer says 32% of SIP fraud incidents come from bad DID routing. That’s not a glitch-it’s a configuration error. Don’t assume your provider will catch it. You’re responsible.

Network and Performance Checks

SIP trunks run on your internet connection. If your network can’t handle voice traffic, calls will drop, echo, or freeze.

• Bandwidth: Minimum 100 Kbps per concurrent call. For 10 lines, you need 1 Mbps upload/download.
• QoS: Enable Quality of Service on your router. Prioritize SIP (port 5060) and RTP (ports 10,000-20,000).
• Firewall: Open those ports. Block all other SIP traffic from unknown sources.
• Latency: Under 150ms. Jitter: under 30ms. Use tools like PingPlotter to test.

Many companies skip QoS because they think “my internet is fast enough.” But if your employees are streaming video or uploading files during a call, voice packets get pushed aside. That’s when you hear robotic voices or dropped calls.

PSTN Sunset Forces the Move

This isn’t optional anymore. AT&T plans to shut down its legacy copper network in the U.S. by December 2025. BT Openreach in the UK completed its shutdown in January 2025. If you’re still on analog or PRI lines, you’re running on borrowed time.

Regulations are catching up too. The FCC now requires all SIP trunk users to have accurate E911 location data-within 50 meters. Your PBX must send this with every call. If you’re using a cloud PBX, it’s often built in. If you’re on-premises, you’ll need to configure it manually.

What Goes Wrong-and How to Fix It

Here are the top three issues and how to solve them:

1. DID not matching format → Check your provider’s DID format. Use E.164 (+15551234567). Match it exactly in your inbound rule. Test with a single DID first.
2. Outbound calls fail → Verify your outbound route has the right dial pattern. For U.S. 10-digit dialing, use NXXNXXXXXX. Add “1” as a prepend if needed.
3. Call drops or static → Test your network. Use a VoIP speed test. Enable QoS. Check for packet loss.

Pro tip: Always test one DID at a time. Don’t try to migrate 20 numbers at once. If one fails, you’ll know exactly where the problem is.

What’s Next? AI and STIR/SHAKEN

The next wave of SIP trunking isn’t just about connectivity-it’s about intelligence. 3CX’s latest update (June 2024) introduced AI-powered DID routing that learns caller behavior. If a frequent caller dials your sales line, the system can route them directly to the right rep-even if they don’t press a number.

And STIR/SHAKEN? That’s the FCC’s new anti-spoofing standard. By Q4 2024, all providers must implement it. It adds a digital signature to each call to prove it’s not fake. Your PBX doesn’t need to do anything-your SIP provider handles it. But if you’re using an old system that doesn’t support SIP trunking at all, you’re out of luck.

By 2027, Omdia predicts every new business phone system will use SIP trunks. The future is all IP. The question isn’t if you should switch-it’s how fast you can do it right.

Final Checklist Before Going Live

• ☑ All DIDs are registered and active in your provider’s portal
• ☑ SIP trunk is registered and showing “Online” in your PBX
• ☑ Inbound rules match E.164 format exactly
• ☑ Outbound routes block unwanted international calls
• ☑ QoS is enabled on your router
• ☑ Firewall allows SIP (5060) and RTP (10,000-20,000)
• ☑ E911 location is set for each extension
• ☑ Tested inbound and outbound calls on at least one DID
• ☑ Failover plan is configured

If you’ve checked all these, you’re ready. SIP trunking isn’t magic-it’s engineering. Get the details right, and your phone system will work better than ever.