I have a constant problem sending a text and it may be related to others in this thread. Regardless of how strong a signal I have, when I hit send I get a failed send and the error message Protocol Stack Error. The signal will return after maybe 15 seconds and I retry until the text eventually goes. I have no apparent problem receiving texts or emails. I checked to ensure that the SIM card is properly seated, but I couldnt get a wedge of paper like others have apparently done. It seems pretty secure to me.
I think Roberwat is probably right about the Operating System. Most of the problem phones seem to be using T-Mobile.
It is a pain in the butt. This is my second Pearl Flip - the first had all kinds of problems and had to be replaced. This is my first experience with Blackberry and I am not impressed.