Not a SIMcard problem
I work in a Mobilephoneshop for a Network Provider, and i had the same problem.
The first trinity that arrived in the shop was mine :)
And this little bas... droped many calls.
I talked with the tecnics from my prov. first we changed the sim, but same problem. Reflashed Rom same problem.
It has something to do with teh HSDPA, evrytime my trinity connectet them, the calls was dropped, the bad thing was, we have a HSDPA antenna in teh shop
Only the change of the devise worked.
Hope was helpfull and sorry 4 my bad english!