Thanks for the replies.

Melanie:

Yes I tried to send the commands all together and it seems to work OK. But I do not know why, since there is no check from the program if the module has accepted the previous string...

I was thinking that the problem might be from the PIC itself side.

harrisonsp:
The command suggested is always included in my programs. Also the response from the module is set to be in numeric mode rather than in ASCII, so the flow is reduced to a zero for "OK".

Still mystery covers the case....

Ioannis