SIP Phone Dropping Calls

I’m having a problem with one of my SIP phones dropping calls.

We have 5 Grandstream GXP2000 phones connecting to Asterisk 1.4.4, and 4 of them seem to work fine.

One of the phones causes problems whenever an Agent is logged into it. The output from asterisk suggests that as soon as a call is forwarded to the phone, it immediately answers the call, then immediately hangs up and an “Auto-Fallthrough” message is shown.

All the phones have the same entry in SIP.conf(with different usernames) and all the phone config settings are identical. All the phones also have the same firmware version

Any ideas as to what could be wrong?