I recently moved to USA for 10 months so I had to get an American number. Can I keep my old foreign number? If so how do I do that?
This is not actually a question about Apple.
The number is not associated with the phone, but with the subscription (your carrier's HLR maintains a relationship between SIM and phone number). Depending on your foreign carrier they may let you retain the number while not in use, or even forward calls to the US phone (at a cost).