I too have been having the same issue on Telstra here in Aus. Calls to HTC and Telstra have had the reply that no-one else is having the issue.
See:
Desire Phone call problem - HTC - Android - Whirlpool Forums
and
Desire microphone not activating - HTC - Android - Whirlpool Forums
for what is really happening.
It seems like denial is all around with "smart" phones at the moment.