If the message does not go through or continues to try to send I would assume it may be a Service Center Number.
SMS (text messaging)
Add service center number
To add or change the text messaging (SMS) service center number, follow these steps:
From the Home screen, select Menu.
Scroll to and select Messaging.
Scroll to and select More.
Scroll to and select Message settings.
Scroll to and select Text messages.
Select Message center.
Message center number: +12063130004
When done changing the provider, select Save.
It would'nt hurt to try sim in another tmobile device to see if you can send or not. Try these steps and let us know