How can a text appear to be from a number that hasnt actually sent it?

We are going through a tribunal with a member of our staff who claims to have been sacked by text, the phone she says the text has been sent from was cut off on the day and for a week after she claims to have received it, T mobile have sent a letter to confirm this but she has shown the message to a solicitor and it does say it was sent from the number in question. Does anyone know how this has been done? Is there a way some one can have a text on their phone and make it appear to have come from a number which it hasnt?