me and a coworker have t600's for awhile now (6 months) and both now have similar problems.
1: when trying to hang up a call they will lock up on the active call screen usualy till a soft reset
2: when inbound call comes in the screen will go to the active call screen and ringtone still plays but you need to hit speakerphone button to answer the call.
3: when trying to use the web it makes a connection then drops it and retrys again.

I have blown out the memory on mine and it still happens even with no 3rd party software on it.

any ideas at all out there.