T mobile Sim in Orange iphone will work?

I've heard it works in orange phones, even Virgin mobile sims do now.

They can share the signal, but not each others handsets (unless unlocked). Orange handset would still need to be unlocked to use a T-Mobile sim and vice-versa.

Mobile handsets will take any sim if it is the right size and compatible with the handset. '3' network sims won't work in a 2G handset, even though if there's no 3G signal available, they 'roam' using 2G.