I think there are a few things, though I may be wrong from just not understanding. The things I use that I think are missing are:
Having the JMP number ring multiple devices and phone numbers at once. (The OG feature of Grand Central)
Call routing: GV rings not just all devices logged into it, but also any phone numbers I’ve associated with GV so I can have it also ring my work number during work hours then ring my home number in the morning and evening but go straight to voicemail at night unless you’re in a specific contact group.
Ability to have a contact group, for example “blocked”, where those calls get sent the error for “this number has been disconnected and is no longer in service”
Anonymous caller ID, or more precisely the ability to toggle my outgoing caller ID on/off.
Call screening (prompting for the name of people not in my contacts list before ringing my device then letting me listen to that prompt before deciding to take the call)
Do not disturb (sends calls straight to voicemail for X amount of time where X is configurable upon enabling)
Multiple/different voicemail greetings based on caller ID
The spam filter is something I probably have to give up, but that is something that I’ll miss.
The other features that appear to be missing also appear to not be necessary, like forwarding sms/mms to email.
In listing these out, I am noticing that many aren’t as important to me as they once were. The most important feature to me, that I think is missing, is definitely the ability to ring multiple phone numbers when the Jmp number is called.
Thank you for trying to clear up my understanding of jmp. I think I have a decent enough understanding and while I’m not sure it is a complete GV replacement (yet) it does seem like it might be the closest thing I’ve encountered.