The BT product guidance says
What options does it allow/show in your account?
- you can pair up to 5 handsets to the Yealink
- that it will automatically link handsets to the number of lines
- that once you exhaust the lines it will add the handset as an extension to the first line
- that you can manage devices to assigned numbers and extensions in the BT Cloud Voice Express option on your account in BT.com
There is even an option to add a separate CISCO desk phone to the BT Cloud Voice Express.
Where do you see this product guidance?
Pairing up to 5 handsets is true if you buy them from BT. Be interesting to see if they state that.
I can't manage my second device on BT.com because they don't recognise the second handset there.