Registering a foreign phone (Istanbul)

At the school I work for, the new teachers starting next year have been asking questions about registering their foreign phones in Istanbul. I'm aware that there are other threads that deal with this issue but the advice is rather confusing! So, I would greatly appreciate it if someone could answer the following questions for foreigners moving to Istanbul from the UK and the USA who want to register a phone here:

1. What paperwork is required? Only the person's passport, or do they need their residency permit and work ID number too?
2. Can you register a phone without a work ID number?
3. What's the process? Go to Turkcell/Vodafone/other provider, then go to bank (or is it the tax office), then back to service provider?
4. Will the phone work permanently after this process? Does it have to be re-registered at any point?
5. How long can you live in Turkey before you have to register your phone?

Thanks for all the help in advance.

Eunice