Nokia 6230i won't send text messages.

A 6230i phone on Vodafone suddenly will not send text messages; it is not
due to lack of signal. One day it was fine and the next a message comes up
saying the text message cannot be sent; the 'sending message' bar runs for a
short time but it gives up rather quickly as if the problem is internal. The
phone is not locked and previously had a Virgin Sim card so the 'unable to
send' was quite a familier message but this is not the same. It has been
working fine on Vodafone for some time so it is not the change of Sim Card.
Any ideas please?