my Sony Ericsson Vivaz wont text, however i do have credit?

[SamuelSeabrooke]

The other day my Sony Ericsson Vivaz all of a sudden stopped texting, the message just sits in the out-box, and every time i press re-send, it automatically says "will re-send at (a couple of minutes later to current time)". I have reset all settings, installed new software and tried the sim-card in another phone and it works fine. However i do not want to send it off to Sony and waist time and money. Please could somebody help me into what settings i need to put it in, or what may be wrong with it and how to fix it. Thank you.