grammatical number - How to write the "plural of plural"?

I'm always confused how to write a "something of something else" in English, especially when plural is involved. For example, what would be the correct way to write the following sentences in English:

There are three phones, each phone is of a different type:

• Three types of phones?


• Three phone types?


• Three types of phone?

Similarly, there are several doctors, and each doctor has a car:

• The doctors' cars?


• The cars of the doctors?

Answer

"Three types of phone" ("Three phone types" is also grammatically correct, but less usual).

"The doctors' cars" ("The cars of the doctors" is also grammatically correct, but clumsy).