Is it possible to configure FreePBX to route any calls into a certain phone number to a single account where the caller does not need to enter any extensions? Thus, to the caller, it is like an old-time phone call where they dial the number and it either gets answered or goes to voicemail.
You mean an inbound route pointed directly to an extension? Yes.
or
… to an extension, even though there is no phone on the extension? Yes.