I found this circuit in a 1979 documentation. The phone must be at least 1k magnetic or crystal (piezoelectric) with a 4k7 resistor in parallel. The FETs can be BF245 or other and the coil is formed by 80 turns of enameled wire 28 AWG in a ferrite rod with socket on the 20th turn for C and 30 turn for C from the earth side. The variable capacitor can be obtained from an unused AM transistor radio.
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