Sony Ericsson S700i

I am having problems sending Text messages. When i push the send button, shorty after that it says "Sending failed". I havent changed any settings or anything on the phone, except for the mobile networks. but i have also changed this back the way it was and i still can't send text. i have T-mobile. you think its maybe t-mobile the problem, not the phone. I sent hundreds of text messages before with this phone.