Could be a few things but if your T-Mobile connection is like mine your assigned an RFC1918 address and NATed to the Internet. It's quite likely that your having NAT issues or even that the IP on your phone is switching at times.

You should look at using a STUN server or other proxy but you can play around with a few things:

- Try adjusting the registration time on the phone to less than 5 minutes.
- Try adding qualify to the configuration if you don't have that already. This will force keep alives to help with NAT timeout issues.
- Make sure the phone is configured with NAT=YES if it isn't already.

Good luck -