Guy goes out on a pseudo date with a girl- maybe it was a date, maybe it was a friendship thing, it was unclear. They go to dinner, drinks, a fun time had by both.
Next day, girl wants to say thank you, after all that's the polite thing to do.
So.... the question--- text message or phone call?
Let's evaluate.... text message- it gets the job done but in a passive more girl-like way; phone message- a possibility but a little more forward, a little more guy like behaviour.
I vote for the text message however, some guys have been saying otherwise.