A plea for help - my father has been sent a short text message from overseas to his 2 degrees number, and it has been delivered over 200 times, and keeps coming!  This is making the phone virtually unusable.  The sending phone has been turned off but the message keeps arriving

I'm guessing there is a receipt not being passed in a server somewhere - can anyone in 2degrees help with this?  Dad has been in contact with the helpline and was advised he may need to change number, but we really don't want to do this because the number is used for business and is printed on cards etc