Strange: Text messages after every call

Archived from groups: alt.cellular.cingular (More info?)

I bought an unlocked T610 phone from a friend and everything worked fine.
Suddenly I get a text message whether I make or receive a call with the
subject "Unknown".

Is this a setting that I mistakenly did or is this a virus? I've heard of
mobile phone viruses.

Has anybody else seen something this strange?

Thanks,

AG