I’m using “follow me” (with pjsip). It is possible to activate or deactivate it from the corresponding internal extension (*21).
Could it be activated / deactivated, too, from an inbound call coming in from voip provider via trunk and inbound route to the same internal extension (inbound route “translates” from the external number (DID) to the completely different internal number)?
Not exactly, but you could set up PJ-SIP extension that you can connect from outside (using a VPN, for example). The problem with an “out of the box” DISA solution is that there’s no way to “become” an extension on the network and disable FMFM from there.
Another option would be to set up a web page and manipulate the FMFM settings through the Asterisk Database directly, turning FMFM from the rasterisk interface.