You can use a unique DID for each analog trunk - or you can use the same DID for all analog trunks - or several DIDs for several analog trunk groups. It's completely up to you and depends on what you want to achieve.
You can call it a fake DID - and why not, it is not a real DID that would come from a digital or IP trunk. But does that really matter? The DID can be any digits you want - it does not need to match an extension number. You could use one digit DID or 10 digit DID - or you could match an extension that is supposed to receive calls from that specific trunk. Again, up to you...
Once there is a DID associated with the call, you can then define an inbound route that will route the call to wherever you want (extension, right group, time condition, IVR, voicemail, ....).