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S100: GSM trunk stops working randomly

Hi. We are having a problem with our GSM Trunk installed in a S100 PBX. This trunk stops working randomly and it appears as busy, as shown in the following screenshot.

Sometimes we get a "Not Registered" status instead which also occurs randomly. Either status, "Busy" or "Not Registered" remain this way until a server reboot.

We have an application server running elsewhere that logs into the PBX. The server has a couple of functions around the PBX:
- Make "Send SMS" requests using the API (sms/send)
- Monitor all trunks statuses, also using the API (trunklist/query)

We checked the logs generated by our application server when this problem occurred. This is the JSON response of the last trunklist/query API call that reported IDLE for the GSM trunk:

{
"status": "Success",
"trunklist": [{
"trunkname": "FXO1-1",
"status": "Idle",
"type": "FXO",
"port": "Span1-Port1"
}, {
"trunkname": "FXO1-2",
"status": "Fault",
"type": "FXO",
"port": "Span1-Port2"
}, {
"trunkname": "gsm-trunk",
"status": "IDLE",
"type": "GSM",
"port": "Span1-Port5"
}, {
"trunkname": "velarde1",
"status": "registered",
"type": "SIP"
}, {
"trunkname": "cbba1",
"status": "registered",
"type": "SIP"
}
]
}

Minutes later a "Send SMS" request is sent to the PBX. These requests work almost always, but this time our application server receives the following response:

{"status":"Failed","errno":"10027"}

Where the code 10027 indicates "The GSM module is in use or there is no idle GSM Module".

Our application server makes another trunklist/query request, and the response is:

{
"status": "Success",
"trunklist": [{
"trunkname": "FXO1-1",
"status": "Idle",
"type": "FXO",
"port": "Span1-Port1"
}, {
"trunkname": "FXO1-2",
"status": "Fault",
"type": "FXO",
"port": "Span1-Port2"
}, {
"trunkname": "gsm-trunk",
"status": "Sending SMS",
"type": "GSM",
"port": "Span1-Port5"
}, {
"trunkname": "velarde1",
"status": "registered",
"type": "SIP"
}, {
"trunkname": "cbba1",
"status": "registered",
"type": "SIP"
}
]
}

And it shows the same status forever. Further SMS requests fail with the same error code. The only way to reset it is rebooting the PBX.

This is a problem we have had since the very beginning of our use of this PBX, but as our infrastructure keep growing the workaround of rebooting the server is each time less possible, and its high randomness prevented us to gather enough information about it (it can take hours or weeks since the last server boot up).

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