That was called rate of return regulation. In the US, only little
rural telcos still do that. Big phone companies have negotiated price
caps instead, which give them a new incentive to invest as little as
possible in the regulated network.

For the most part, mobile phone rates aren't regulated at all.

You are confused. The cell phone companies are in a different business.
Verizon may own a public telephone company, but most of the US has public
phone companies owned by someone else. The phone companies providing
landline phone service are still regulated entities regardless of who owns
what.

Yes, they're regulated, but most of them are under price caps, not
rate of return. So long as they don't exceed the price caps, the
regulators don't care what their capital investment or profit is.

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