Expert seeing "This phone number belongs to a landline..." invite error

If you see this message: 
This phone number belongs to a landline. Try again with a mobile phone number.

That means: 
Phone number used is a landline, not a mobile phone number. This number will not work for a Streem call. 

What to do:
Please ask your customer for a mobile number (see our supported devices for customers) OR use a different communication method and our "Link & Code" method to send an invitation.