> can a single phone have radios for all the different carriers?
All carriers in a single country? Yes, many do. All carriers worldwide? You'd be putting a lot of radios in the thing.
> Could they share a single antenna?
Not a radio expert, but I think each antenna has to be tailored to each wavelength. Maybe you can get away with one per band (one for 800-900, one for 1800-1900, one for 2100-2600)
> Could one be activated by the user selecting which network to get on?
You'd need a SIM card for each carrier you want and swap them out, and for American CDMA carriers you'd have to have your ESN registered with their networks beforehand.
All carriers worldwide would be tough, but a sufficiently multi-band GSM phone will typically give you pretty much worldwide roaming from a variety of carriers.
I'm in Europe. The last time I had a phone I couldn't effortlessly take to the US or Asia and use roaming was probably about '99.
Yeah, most modern phones will work most everywhere, but there'll always be one carrier's 3G or 4G network you'll miss out out. Some phones have really impressive radios in them now. Here's the Sony Xperia Z Ultra:
> You'd need a SIM card for each carrier you want and swap them out
There are phone models that support 2-3 SIM's , very useful when travelling so you don't have to swap anything but can call from different networks seamlessly.
All carriers in a single country? Yes, many do. All carriers worldwide? You'd be putting a lot of radios in the thing.
> Could they share a single antenna?
Not a radio expert, but I think each antenna has to be tailored to each wavelength. Maybe you can get away with one per band (one for 800-900, one for 1800-1900, one for 2100-2600)
> Could one be activated by the user selecting which network to get on?
You'd need a SIM card for each carrier you want and swap them out, and for American CDMA carriers you'd have to have your ESN registered with their networks beforehand.