My granddaughter spent 3 years living in Italy and bought a samsung slide type mobile phone, it is on the Italian "TIM" network however it is unlocked, today I put in a tesco mobile sim card and it works with that sim, my question is if I make a call on it with the tesco sim does the call go all the way to Italy and back here to connect ?
Your service provider is the one who provides the SIM card and to whom you pay on a regular basis. The handset is just a handset. If it reads SIMs and operates on a locally used frequency then that's the limit of it's involvement.