John Farris – When Michael Calls (New English Library, 1970)
“How could a series of phone calls… terrorize an entire community and lead to murder? Well, for one thing, because Michael Young was supposed to have died sixteen years ago. Who was calling then? Was it some fiendish prank, perhaps one of the boys from the nearby school for disturbed children? To Helen Connelly it seemed uncannily like the voice of the nephew she remembered so well. Besides, this Michael, whoever or whatever he might be, knew things about her that only the real Michael could have known!”
Thanks to Calenture for the cover scan & Steve Goodwin for the blurb