I have been given a samsung jet mobile....really nice phone its on vodafone but i have put an o2 sim into it and phone found the network but it only allows emergency calls only.......is it blocked due to being lost or stolen ?

It probably is locked to the Vodafone network, try googl'ing around for an unlock to your exact model! Once you've found one, try it out!

This will probably solve your problem, if not, try contacting your network supplier (O2 in your case).

DFK