Nokia E63 Incoming call stops randomly

[paramvirs]

i have a nokia E63, it works fine for some time like 6 hrs and then it stops receiving incoming calls from FPL account.
All the calls then go to my voicemail. I checked and saw that SIP profile was shown as connected to FPL.
I tried an outgoing call and it worked fine.
I again tried incoming call but it went to voicemail.
Now i re-registered my SIP profile, i mean i changed its setting from "Always on" to "when needed" and then again back to "Always on". It solved the problem.
But after few random hours mostly more than 4 hrs, incoming calls again stopped working.
Rebooting the phone also solves this problem.
Can someone help me out here?I need phone to be always on and always ready to receive calls.

Here is the process i used to register my phone to FPL http://forum.freephoneline.ca/viewtopic.php?f=15&t=1046

[Bloodsong]

First thing I can suggest is that you look for a "registration expiry" setting. It's usually defaulted to 3600 (I assume seconds), which would be 1 hour, but several people who have this sort of issue have found that dropping it to something around 600 has helped.

[paramvirs]

thanks for the suggestion. can someone also suggest that where i can find this "registration expiry" setting in my nokia E63 phone.
I have even installed "Sip voip settings" program but cant seem to find "registration expiry".
There are other settings like
TCP NAT bind refresh=0, UDP NAT bind refresh=0, CRLF refresh=ON for FPL profile OR
TCP NAT bind refresh=1200, UDP NAT bind refresh=28 for my wireless network connection.

[paramvirs]

i had solved this problem. Eventually the problem was created by me few months back when i was struggling to get FPL working on my phone.
I installed "SIP VOIP Settings" utility and unders NAT Firewall Settings -> My FPL profile name, i had changed the values during testing.
Now i deleted this "My FPL profile name" and re added it.
The cellphone is working fine now.No missed incoming calls.

Thanks all for their support

[Bloodsong]

Ah, I'll have to remember that in future, not too many phone like devices where someone thinks to check firewall type settings.