Grandstream GXP1405 Hangup detection

I am new to FreePBX. I have installed it in a DigitalOcean machine and use it in my home.

It works fine, but sometimes I call my office extension and it is occupied when, in reality, it is not.

I guess sometimes the PBX does not recognize the hangup and it keeps the extension busy.

By the moment I am using it only internally with no trunk to PSTN, so it is something in my FreePBX settings or in my Grandstream GXP1405.

I have researched the forum on issues about hangup detection, but most problems seem much more complex than mine. I would be glad if someone could point me in a direction to solve this or at least detect the specific scenario that make it happens.

Thanks,
Fred


FreePBX Version = 2.11.0.37
Running Asterisk Version = 11.10.0
Operating System = CentOS release 6.5 (Final)