To oversimplify:

You can say whatever you want on a telephone call.

BUT:

The telephone network is regulated. Your cell phone must comply with FCC regulations. You personally may have a restraining order that prohibits you from calling certain people.

IE, if a phone is found to violate FCC rules, pulling it from the market has little to do with the first amendment.

If these FCC rules were designed specifically with the intent to suppress speech of certain parties, they could be found in violation of your first amendment rights if challenged. IMO the ruling does not bother to examine whether the motivation of drafting the Act was to suppress speech.

The FCC doesn't make rules based on who owns the telephone though.