josejb70, I have a SonyEricsson T637 as well, and have experienced the slowness with the phone on occassions. I have not had any problems with receiving and dailing using the T637, but have noticed the slowness when pressing the talk bultton on the steering wheel and nav unit.

What I have learned is that pressing the talk button on the steering wheel or nav unit normally takes a second or two for it to work. Just have patience and the call will go through.