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?

Solution 1:

"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).