Would it be possible for a cell phone to change networks at will?

Some cell phones have slots for several SIM cards. Could this mean that you use cards from different providers and pick the one with the best service wherever you happen to be?

You select which SIMs to use; they are linked to different phone numbers. I did that when I had a Spanish and a Swedish SIM for example. If I had gotten a call for my Swedish number while in Spain, the phone wouldn’t have used the Spanish number (I still would have gotten those roaming charges). Or you can for example carry your “work SIM” and your “personal SIM” and connect one, or the other, or both at the same time.

In theory and depending on which agreements they have between each other, the phone should be able to pick the best signal for whichever of the “partnered” carriers is around, for any phone.