This circuit makes use of an optical coupler to isolate the telephone line. The circuit is from 1970s documentation. The components admit equivalents.
This low power circuit can be powered with voltages from 1.8 V. We find it in a 80s Hands On...
This simple unijunction and bipolar generator appeared in Radio Electronics magazine in September 1865....
This circuit was obtained from an old American documentation having a limited adjustment range....