So, I currently have a UK O2 iPhone contract. It's a good deal which even includes includes free roaming texts.

I also have a CoopMobile pay-as-you-go SIM and a old beat-up Nokia.

I would like to keep both my current UK and CH numbers, but be able to use an iPhone decently in both countries (ie. all proper iphone services like internet, voicemail, etc.)

Does anyone have a good solution for this?

Something like swapping the SIM depending on the current country I'm in. Then the currently unused SIM forwards calls (maybe via VOIP?) to the other SIM.

There seems to be a lot more choice for iPhone suitable contracts in CH, a bit more confusing to choose. Does anyone have recommendations for a service that I can drop into my current iPhone? Should I consider network with regard to coverage? I would be mostly using the network to/from Gstaad/Geneva via road or train.

Anyone experienced with dealing with O2 half way through a contract? If possible I don't really need to be spending a whole contract's worth in the UK. Would just need for the occasional visit where local data usage would be handy. Something like moving my UK number out of contract onto a pay-as-you-go (including data) and moving a different number (like friend or family) into my current contract.