Random external did calls disconnect after few seconds

Good day,

Am new to the pbx world and have a puzzling problem. Have three sites running with polycom vvx310 phones. Incoming DID go to ring groups. I have one site that on random occasions the call seems to drop just after being answered. This usually last for about 15 to 20 minutes. The caller experience is that the person answered has hung up the phone which is not the case. Any ideas?

Should mention i had submitted a ticket to our DID provider and they said they see that the Pbx has sent the hang up request.

Running Asterisk FreePBX