There is no build in limitation to disconnect the call after 15 minutes. So I assume there is a problem in negotiation of some keep-alive mechanisms. 
What version of PhonerLite are you using? The actual version is 3.20. At "configuration/network" there is an option called "Session Timers". You may try to deactivate this and save.
Via help menu you can also enable the debug window where all sent and received messages can be seen. You may send the content of the debug window by e-mail (not here in the forum), so I can take a deeper look at that problem.