iMessages app on Mac(Mojave) accessible only by email address not telephone number?

I'm not sure where I went wrong....

I've setup a new to me MacBook Air, it has been upgraded to Mojave ... the Messages app only sees and uses the email address for messaging. The associated AppleID includes both the email and telephone number. It works correctly with an iPhone and the same AppleID.

In the iMessages preferences - settings, the "start new conversations from.." is greyed out. No where on the Settings page does it show the telephone number. And the email address that is associated with the Apple ID is checked but also greyed out and can't but unchecked.

I have signed out and signed on... and restarted. Since the telephone number is already associated with the Apple ID and working with an iPhone, is there a way to add the telephone number to the MacBook so that it can send/receive texts to the email and telephone number?

any thoughts?

thanks in advance.