Help with a V600

Bought a V600 out of the free ads last week.

Put my Orange contract simcard in it, works fine. And Orange PAYG works OK too.

Put another sim in (Voda or O2) though and I get a signal and can send a text, but can't make calls. It dials the number, then 3 descending beeps and 'call failed'.

Thought about flashing it - maybe the Orange firmware on the phone is causing the problem.

Or is it something more sinister? If it was blocked it wouldn't work on Orange... or would it?

BTW if it is blocked I am not looking for a way to 'unblock' it....