If you are sure the phone company is disconnected, You likely can send as much as 200 volts at one amp to close a relay to turn on the pump. 80 volts is 80 watts at one amp. I'm assuming your relay is designed for 120 volts at one amp control current which will only warm 200 feet of wire slightly unless the ground is better thermal insulation than usual. If the relay control voltage is lots less than 120 volts, you probably cannot use that phone line, reliably. 120 volts is likely a code violation, but will probably work if you don't mind the slight risk of a fine. Pushing your luck, you can use all the wires in the cable as the hot wire and use grounds rods for the negative ground path. Alternately solid state and hybrid relays are available that pull in with less than 28 volts and less than 0.01 amps. Possibly they will call the relays, contactors. The pump contacts need to be rated for the starting surge of the pump motor. Possibly 50 amps.

Most phone wiring is quite thin gauge...about 26 AWG is typical. I would be inclined to use some sort of active circuit at the distant end to detect the presence of about 30 VDC to activate a pilot relay. If there is a pump out there, there is electrical power for it.

The ring voltage is 20 Hz (60 Hz countries) at a few tenths of a watt.

Don't do that ! You might overload the phone line DC power source and might receive a costly repair bill from phone company after they find out you were misused their phone line.

Phone uses 48V to 50V DC power source, not 90V.

Keep in mind that phone line voltage is DC.

You use a gauge calculator for current capacity. Lots available online.

You can parallel both sets of wires and send about ~1A max. @~ 32 V

4?/ 200ft.

You may want to put it inside a conduit, like plastic drip irrigation tubing, for moisture resistance and bury it.

Previous answers noted a ringing phone uses 90 volts but of course with very low amperage. I would like to use an aleady buried phone line (four wires) between my house and barn about 200 feet to activate a relay to start-stop a submersible pump.'