mobile refuses to send texts!

[bossychloe]

I gave my niece my old phone (Samsung D500) she replaced my virgin pay to go sim with her vodaphone pay to go sim - but whenever she tries to send me a text it comes up as 'error'. I have re-checked my number and ensured she has money on her card but it still refuses to let her text! Any suggestions?
Thanks.

[REDHEAD24]

is it just when she txts u she gets that - is the sim in propely?

[solera]

Is your old phone locked to Virgin?

[bossychloe]

Thank you both for your replies.
I have had her sim in and out a few times to make sure it is in properly so I don't think that is the problem.
I don't think my phone is locked to virgin - how do I check and how do I unlock it - sorry to sound so dim...
please help, we are tearing our hair out!