I've found *73 to be the solution, twice in the past year, as is only rarely mentioned/indicated in any of these posts(multiple threads / google search hits) about the same issue. Particularly, in my case, I began to experience this when a Webex meeting would try to call me using the "callback" function.

Specifically, as of Sept.2019:
Has to do with "immediate" and "conditional" call forwarding(even though it may not have explicitly been chosen or desired) and can be disabled by:
Dialing *73 then press send... That's it, resolved.