I’m not disagreeing that it would be strange for a SIP service aimed at PABXes to limit calls to one per caller ID. What I’m saying is that this strangeness isn’t something new to SIP, it is how business numbers have worked for many decades, from long before the internet.
I’m also challenging a further extension of the misuse of “DID” in the VoIP world.
Historically, a business might have had dozens of lines in a a hunt group, all of which had the same directory number and presented the same caller ID. The term DID was never applied to such a directory number; it was just the phone number of the PABX.
With real direct in dialling (in many cases DIDs are only direct in dialling to the provider, not to the customer), you might have, say 50 lines and a block of 1,000 numbers, if you were a big user. Incoming calls on any of those 1,000 numbers would hunt over the 50 lines. Outgoing calls, in the early days, would also hunt over the 50 lines, and would present a pilot number, normally the first in the range of numbers, although ISDN made it possible to present other numbers, and many big users chose to not present one at all, as they couldn’t tell where to send returned calls.
With SIP, what you sometimes get is accounts with a limit of one simultaneous call, but it is the account, not the directory number that has that limit. Such accounts are not intended for PABXes, but for single phones.
It is possible that, if you try to use multiple such accounts with FreePBX, FreePBX has difficulty telling them apart, so they look like one trunk, rather than a trunk per account, which is maybe what is happening here.