Calls cannot be connected as equipment may be dis connected?

Hi all,

Every couple of days when people are trying to ring through to freepbx from the public network they receive a message telling them the call can not be completed as the equipment may be dis connected. A reboot of the server corrects this until the next time it happens.

As I’m a novice when it comes to freepbx can someone suggest what is going on or where to start looking for problems?

FreePBX is configured with a BT SIP trunk
Asterisk (Ver. 1.8.11)

thank you kindly