Why does an IP PBX stop receiving calls without an alarm?
Because the problem is rarely in the PBX. On an office Intelbras or Grandstream IP PBX, the trunk depends on the site’s router and internet link. A registration that does not renew, a router with SIP ALG or a public IP that changed takes inbound calls down, while the PBX stays on and extensions keep calling each other.
The failures that show up most often:
- Expired registration. The trunk loses its SIP registration after a link outage and only comes back on the next attempt, or not at all if the password or domain changed.
- SIP ALG on the router. SIP ALG rewrites the SDP and causes one-way audio or dropped calls.
- Dynamic IP. On broadband, the public IP changes, and a carrier that authenticates the trunk by address starts rejecting calls.
- Codec. A trunk configured with G.729 only, or a carrier that only accepts G.711 A-law, ends in a 488.
- Channels. The trunk’s simultaneous call limit shows up as a 503 at peak times.
Where do you look for each symptom?
Intelbras IP PBXs and the Grandstream UCM line show trunk status in the web interface and keep call records. The site’s router is the second place to look. 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 after a link outage | Expired registration, or 408 Request Timeout on the REGISTER | Trunk status in the web interface |
| Outbound rejected | 403 Forbidden: IP or credentials not accepted | The link’s current public IP and the carrier’s records |
| One-way audio | NAT or SIP ALG | Router settings, RTP ports |
| Rejected right after dialing | 488 Not Acceptable Here | Trunk codecs |
| Rejected at peak times | 503 with cause 34 | Active calls against the contracted limit |
| Call drops after about 30 seconds | ACK or response lost to NAT | Packet capture on the PBX, if the model supports it |
What does a real test call catch that the PBX interface does not show?
The PBX interface shows what the PBX knows about itself. It does not see calls the carrier failed to deliver, or calls lost in the router before they reached the PBX. A call placed from the outside, like a customer’s, shows the failure as the caller experiences it, with the SIP code returned.
It also checks the rest: whether the right greeting played, whether audio flowed both ways and whether the measured voice quality stayed above the agreed threshold.
What about a branch with no IT staff?
That is the most common case for an office IP PBX: nobody looks at the PBX until a customer complains. Monitor calls at the agreed pace, including at night, and notifies the responsible team by email and SMS. Paired with the site’s internet link, it tells a voice outage apart from a connection outage.
Internet link monitoring uses an agent 2N5 installs on site, with your written authorization.
How does Monitor monitor an IP PBX?
Monitor does not integrate with Intelbras or Grandstream IP PBXs; it calls the numbers, so it works with any of them. 2N5 retries each failure before alerting and delivers the incident with the SIP code and Q.850 cause of each call.
See the details in SIP trunk monitoring.
Frequently asked questions
Does Monitor work with my Intelbras IP PBX?
Yes. Monitor does not integrate with Intelbras or Grandstream IP PBXs. 2N5 calls the numbers you choose, the way a customer does, so it works with any model and firmware version, with a SIP trunk, E1 or analog line, and with any carrier.
Why does the trunk drop when the branch's internet link flaps?
Because registration and media travel over the link. A short outage can leave the trunk unregistered until the next renewal, and a new public IP makes the carrier stop recognizing the trunk when it authenticates by address. Inbound calls during that gap are lost with no error on the PBX.
Do I need to turn off SIP ALG on the router?
In most cases, yes. SIP ALG rewrites SIP packets and the SDP, and the usual result is one-way audio, calls that drop after a few seconds or unstable registration. Turn off the ALG and use the PBX's own NAT options, with the RTP ports forwarded.
What if the PBX also has E1 or analog lines?
Each inbound number is a voice circuit, whatever the technology. The test call dials the number and judges the result, so an E1 or an analog line is monitored the same way as a SIP trunk, with the response and Q.850 cause the route returned.