False alarm. This turned out to be a medical appointment reminder service that apparently uses auto-dial/robo-messages but lacks proper caller ID information.
First call, three rings, didn't recognize (misspelled "East Falmouth" appeared on caller ID), hit BLOCK NOW. Immediate call-back from this same number, now successfully blocked. Within seconds, an apparent third call, now -7015 (changed final digit) which I allowed my answering machine to pick up. Result: a constant, very loud mid-frequency tone during the answering machine's greeting, then a hang-up at the "record" tone. A new kind of overly-aggressive robo-call?
False alarm. This turned out to be a medical appointment reminder service that apparently uses auto-dial/robo-messages but lacks proper caller ID information.