Want to restrict texting

I wasn't sure where to post this, as its not really phone brand/model specific..... so I guess the Lounge will do :)


I'm trying to restrict the use of a phone to the sending of texts to one specific number.
I tried turning on fixed dialling (this was with a N3510, 6230 & 7260) and restricting use to one number, this works fine in that it prevents the phone from calling any number other than the specified one but it will not allow it to send text's to it.

This is for a business phone which is used by staff of a texting service to send texts to themselves to test/troubleshoot the system, however some staff are abusing the situation by using it for their own texting/calls, this is why it is desired to restrict usage to one number.

Anybody got any suggestions?