I used my AH Business account, PostmanPot used my AH Business account... and we were both using it at the same time.
So much for "locked to the phone number of the subscriber".
I also gave Zertop my account to use as a test, as well as a couple of other friends after that to test... trying to eliminate my setup as a variable, this was before the official IPC announcement or acknowledgement of any issue.
Maybe this too is one of those "grey areas".
This definitely should not be happening. It could be that if you upgraded from a Capped account in recent month, the account inherited the traits of it's predecessor and allows concurrent connections, which should have been updated by the Radius when you moved data on the new profile setting.
I would be truly grateful if you would PM the username concerned. If there is a bug in the system, it may be something we can sort out to improve experience.