I am currently with Virgin here in the UK with an international roaming SIM - I pay as I go. When In France (I go about every 3 weeks) the provider will change wherever I am travelling - orange, bougeytel(sp.) and others. Often they will change in the same area. My wife has exactly the same set up but is usually on a different provider than me!
Calls are not cheap when compared with domestic calls and of course you pay for incoming texts and calls too.
We have often talk about buying cheap pay as you go phones for when we are in France but we simply do not make or take that many calls.