I think the BT Business Cloud Voice Express is locked down to their devices.

I cannot manually add a phone or device that is not purchased directly from them,

It has to be added via the CVE dashboard and it uses the MAC code.

I just saw my ATA 191 that I bought from BT appear before my engineer visit tomorrow, I assigned it and now I cannot login with default admin credentials.

All you need is the proxy User ID and password
all other ids can be left as default.
user id needs to be entered in the AUTH ID as well but the first picture page will fill that in for you.

whether BT will give you these details is another question
the ATA191 they sent you has probably had its settings changed so you cannot login and get the details you need.

Only BT can give you the details as they are linked to your account

My "engineer visit" is today 8-1.
It appears that the provisioning has been completed remotely as my alarm and old cordless phones are working.

I cannot get into the ATA191 to see the settings which is a bit of a pain as I paid for the device, it is not bundled with the contract as with the voip Yealink phone.

I will have to ask BT to give me the details but I bet they won't.

I did backup the initial config file when I got the device with plenty of BT URLs. Don't know if it will be any good as the mac code of the new 191 is tied to the CVE account.

I might sell the 192 or keep it and try on another network.

Engineer just attended and said I can't login to the device as everything has to go via BT.
He didn't do anything as I already set everything up.

Given its BT, I’m not at all surprised it’s all locked down and they won’t give you any details.

The ATA on boot will login to the BT cloud and download any settings to configure it automatically the engineers know nothing about the settings.

Most likely.
When I purchased my own ATA 192 I had to google how to access the device, login and manually change the settings to add it to my network. The 191 I just plugged it and added to my account on the day of the visit and it was working.

I do want to ask BT how I can change the ring delay as the phones connected to the ATA are delayed by 2 seconds. No biggie but annoying.

There is a ring delay with voip as its the way the call is set up being internet based when you dial the number it then sets up the path (unlike a phyiscal phone which has a static path) the voip phone doesnt so you then get a delay think it would be better if you didnt get the ringing tone on the phone until the phone was actually ringing but.