Actually, the phone company is still considered a utility in most places, and there are strict rules on what they can and can't require or enforce, so that's a bad example. Ford provides a product, not a service, and you have both signed a contract detailing the terms of that agreement.

It's a little different with ongoing unregulated services. They do have the right to provide services based on their own choices, within the law. And if a change is made after the service is used, the user's choice from that point forward is to either use the service in accordance with the TOS of the company or not use the service.

So, yes, unless they are regulated by some government entity (like utilities, broadcast stations, etc) they do have the right to change the terms of service when there is no pre-existing contract saying they can't. The TOS states that they can change the TOS at their discression.