I am having a problem with T-mobile. When I do gprs I can not go to google.com.

I started playing around with my MTU settings in /etc/ppp/options, I set the mru to 512 and mtu to 576, I can now get to google websites, woohoo.

Well why am I telling you in this thread?

OK, now googletalk will not sign in. I also had same issue using a proxy server googletalk would not sign in. Somewhere along the path, whether its your router or out on the internet somewhere. See if you can find a way to play with you mru/mtu settings and I bet you will find a solution that works.