I am having the same problems with my 8700G. I contacted T-mobile and they are fully aware of it. The engineers are working on it right now. It is very frustrating for me. I keep getting messages saying communcation failed or network failed while I am in the middle of a conversation with my friends.
I guess we need to wait until the problems are being resolved.
If there is another better way to this, please let me know!
I highly recommend paying the 19.95 and downloading the newest version of Yak-On OTA. yak-on.com from your Blackberry Browser. For some reason with T-mobile, Yak-On can crash if you try to do the desktop install.
The OZ client is pretty bad, and is definitley not acceptable for anyone who has tried using AIM on the sidekick or similar.
IMHO the yak-on client is better than the sidekick2, tabbed windows really makes the difference for me.
If you are going to use OZ make sure you are ready to pay a lot for SMS messages. OZ uses SMS to "chat" and you are charged by T-Mobile for every incoming and outgoing AIM message. If you are going to use OZ you may want to buy a block of text messages from T-Mobile or go with their unlimited plan. If you want to save some money check out Ramble IM. It connects directly to AIM and does not use SMS so you do not pay extra. It works off of the Blackberry internet connection.