So... I've just purchased a Oneplus 3T. This is a dual sim phone, although that isn't why I purchased it. It seems a shame not to use it and I currently have an O2 sim in along with my regular contract and it's nice having 2 numbers.

So my question is... Is there a sim I can keep in just for emergencies that doesn't need regularly topping up? Or an international roaming sim? Any suggestions?