How does a SIP trunk on 3CX fail without warning?
3CX tracks the trunk’s registration, but registration is only one part of the call. The carrier can accept the registration and reject the calls, the router can break the media, and a changed inbound rule can send the number to the wrong place. In all of these cases the trunk still shows as registered.
The failures that show up most often on a 3CX:
- Registration that drops and comes back. A password changed at the carrier, a DNS change or a new public IP leave the trunk without SIP registration for short periods, and inbound calls during that gap are lost.
- NAT and SIP ALG. A router with SIP ALG turned on rewrites the SDP; the typical result is one-way audio.
- Codec list. A codec order the carrier does not accept produces a 488.
- Simultaneous call limit. The limit of the contracted trunk, or of the license when your edition has one, shows up as rejections at peak times.
- IP ban. 3CX’s attack protection can block the carrier’s IP after repeated authentication failures.
Where do you look in 3CX for each symptom?
The first place is the Activity Log in the management console, with the log level set to Verbose while you reproduce the failure. It shows which trunk took the call, the carrier’s SIP response and the rule applied. The table links each symptom to the most common code and the point to check.
| Symptom | SIP code or cause | Where to look |
|---|---|---|
| Inbound stopped | Registration lost, or repeated 401 Unauthorized | Trunk status and Activity Log |
| Outbound rejected | 403 Forbidden: credentials, IP or caller number | Trunk settings, outbound caller ID |
| One-way audio | NAT or SIP ALG | 3CX public IP, RTP ports on the router |
| Rejected right after dialing | 488 Not Acceptable Here | Trunk codec list |
| Rejected at peak times | 503 with cause 34 | Active calls against the contracted limit |
| Number does not exist on outbound | 404 Not Found or 484 | Outbound rules and dialed number format |
What does a real test call catch that 3CX does not show?
3CX does not see calls that never reach it. If the carrier stopped delivering your number, or delivers it somewhere else, the dashboard stays green and the log stays empty. A call placed from the outside, like a customer’s, shows the failure as the caller experiences it, with the route’s SIP code.
It also checks what comes after the answer: whether the 3CX IVR played the right greeting, whether audio flowed both ways and whether the measured voice quality, MOS-LQO, stayed above the threshold.
How does Monitor monitor a 3CX?
Monitor does not integrate with 3CX; it calls the numbers, so it works with any platform. 2N5 places real test calls at the agreed pace, retries each failure before alerting and opens an incident only after consecutive failures, four by default, with the SIP code of each call.
The alert follows the escalation ladder by email and SMS. See the details in SIP trunk monitoring and, for service numbers, in 0800 and IVR monitoring.
Frequently asked questions
Does Monitor need access to the 3CX console?
No. Monitor does not integrate with 3CX. 2N5 calls the numbers you choose, from its own platform, the way a customer does. That is why it works with 3CX on-premises or hosted, in any version, and with any carrier.
Isn't the green trunk indicator in 3CX enough?
The indicator shows the trunk's registration state. It does not tell you whether the carrier is delivering calls to your number, whether the inbound rule leads to the right destination, or whether audio flows both ways. A registered trunk can reject every outbound call with a 403 or a 503.
Why is audio silent in one direction on 3CX?
Most of the time, because of NAT or SIP ALG on the site's router. The router rewrites the media addresses or does not forward the RTP ports, and one side's voice never arrives. Turn off SIP ALG, and check port forwarding and the public IP configured in 3CX.
Do the test calls use up 3CX simultaneous calls?
Yes, for a few seconds, when the tested number comes in through your 3CX. The test call is an ordinary inbound call, within a call limit, and goes to the number you chose, such as an IVR, a test tone or an echo service.