Lost Mobile Phone cannot be removed from system

I had a similar experience but lucky enough to get it resolved (but after a month of hellish experience). It took about a month to get it resolved though. In my case, the device didn't have a problem but bought a new device (when iPhone 15 was launched; old was 14 and I gifted away the old). The number from the old device couldn't be ported over to the new device, but CS suggested to activate the new with a new number (realized later this was a very very bad advice), then cancel and port over; well, that didn't work; with the new line cancelled, the new device vanished from the system/account. So, was with a locked device with no phone line (only wifi), cannot use it with Cox, cannot use it with any other carrier and no other phone number to use. Within couple of days I realized this was going to be bigger issue to resolve, so I ported my old number to another carrier temporarily with an old device (iPhone 8) that I had so that I had a functioning phone number.  It was funny that I was carrying two devices wherever I go for about a month.

The store support, phone support or the chat support weren't able to do anything (I have to say they really really tried; don't think its their fault; their system has issues or not mature enough to handle these); I think it was an issue their backend tech support had to fix. Every few days, I was on the phone with their support or with them in the store. Eventually after a month, several trips to the store, n number of chats, phone calls, their backend support fixed the issue, resurrected the new device as a brand new device for a fresh activation.  This time I was able to activate, port over the old number to the new device.  I still had to talk to their billing dept to get credits for phone line that didnt work. Thats another thing I have to resolve but was glad that the phone number porting was finally resolved.