Server un-Registering Mitel phones ? Please help

Have just rolled out our new pbx system was running asterisknow 1.0 without freepbx.

Now have Asterisknow 1.5 with upgraded asterisk to 1.6 and Freepbx updated to 2.7.

Anyways the system is running very good, very pleased with the Freepbx project!

Have a weird problem in that being that sometimes out of nowhere my Mitel 5215 phone will just not register, not all of them usually just one of them will no longer be registered. It appears to be knocking the phone offline for what ever reason.

The only thing I can come up with is that the phone server is expiring the registration after such a time.

On the Mitel phone themselves the duration is set to 180 seconds why the phone server would limit it lower then that I do not know.

Anyone have any insight upon this as to why or where I should even start to try to troubleshoot this problem. Any help will be appreciated.

Thanks in advance,