Anyone come across this error message?

This happened to me four times on Saturday.

When I try to send an SMS msg, I got an error in my Inbox/Outbox that read "Protocol Error" with a red cross (X) next to it.

When I try to make a call, I get a message that reads "Call Failed".

I would just pop the battery every time, and we were back in action. But like I said, happened four times on the same day.

Would this be a firmware problem?
Or the handset's fault?
p.s. I did upgrade to the .64 OS last week