Stupid question, sorry...

If I have a problem with my Motorola V3, on Vodafone, which I got from
Phones4U highstreet store, should I take it to the Phones4U shop where I got
it, or to the Vodafone store (since I'm on their network)?  The problem is
I'm unable to use MMS, and I've exhausted every line of enquiry besides
going to the shop. (Including RTFM and calling Vodafone support line).  I'm
confused about 'who' is responsible for helping me out here;  the phone
vendor or the network?

Thanks
Owen