Samsung D900 Not sending SMS!

Ok... i used to have a SAGEM GSM Cell Phone that couldn't send SMSs after some months... now i bought a Samsung D900, changed the GSM Chip to the same of my last phone and i just keep getting an "Error" message when i try to send any SMS. The Phone is perfectly working on calls and even receiving SMS well... but can't send. I don't know if this is a problem with the Chip, the Provider or any configuration on the Phone. Any sugestions?

Thanks!