It used to be the case that MBB contracts did not include a capability to send SMS messages, although there was a capability to receive them, since this was required in order to receive service messages from Vodafone. Logically you might expect the same to apply to MMS, although I have no clear recollection on this point. I think that maybe some other networks had provisioned their MBB service to include outgoing SMS, but my memory of this is even vaguer!
I can't see a specific ban in the current Ts&Cs, but at the same time there is no provision in the price plan for it.
Have you actually tried sending an SMS, or is it simply a capability that the tablet has, but it requires a SIM that also has the capability? If you haven't already tried it, you could try sending an SMS to a conveniently available phone (so that you can see whether it gets through); if it does work, do you get charged for it? Also, if it does work, do you receive any warnings that you shouldn't have done it??
Edited to add: Just seen @AnnS's post. It sounds as though things may have moved on from the situation I describe!
