If you have a situation where handsets are capable of registering but then keep going
offline repeatedly, a quick way to attempt to resolve this is to make sure the "Server Expires" (ref. 2.) setting on the account tab is set to 120 seconds.

You may also want to try changing the “Keep Alive Type" (ref. 2.) to “Options” and the “Keep Alive Interval (Seconds)” to 30 seconds (ref. 3.).