As you said, the cost of the equipment would be only a couple or few hundreds of dollars.
I wrote about the "Craig system", the cost of a triangulating system would be much higher.

The way to avoid the outside "interference" is quite simple

1) Do not switch the rate when such a zone is entered from outsize the zone - as already applied to calls leaving "local zone" (OK that allow you to walk in while speaking - a minor inconvenience)
2) Today if you are out of "zone" you are, when calling out, asked "do you want to call at this fee" and when called asked "do you accept". This could easily be expanded to noisance (I misspelled accidentially, love the "word" I wrote) fee areas. Of course, if outside, you would have to walk/drive a few feet to see that message disappear - so what.

As to the multiplier of the $0.31 that multiplier also apply to the number of people that has to pay for it.

Re older phones: whatever you do older phones will not have it, but the replacement rate of cellphones is astonishing and I would rather have a system universally applied (low codst) that gradually starts to work than a system (high cost) that only get implemented sparingly.

Erik