What is best set up for cheap or unlimited minutes on PAYG to only 4 other mobiles on same or other networks?

I want to acquire a fone (just for making outgoing calls to, say, a monitoring of central-heating type commander system etc,), on a PAYG tarrif- I need to be able to make frequent, cheap, on a bundle say, I sleuthed (Asda) 500 min per month for �20 and---- O2---- unlimited calls, for �15 per month to other O2 fones. Those packages seem the best, but have I missed something you fone wizards out there?? ( I'm a very old lady) Also if I go to an O2 shop will I be able to buy 3 or 4 ready to use Sim cards for using in the fones I am making calls to so as to use the O2 goody unlimited bundle? Will this trigger itemised billing?