Adding Common Area Phones to Speed dial groups

Hello,

I am setting up a CAP and want to add it to the Speed Dial group of my Teams App. 

If I try to add it as a speed dial I can find it and select it but the Add button doesnt work.

If I try to add it to one of the groups I have made I get the error "We couldn't add a mate to the group" 

Should this work or am I trying to do something unsupported?

Thanks in advance

Nigel