Expert seeing "Unable to deliver invitation..." invite error

If you see this message: 
Unable to deliver SMS invitation (spam filtering or restricted number). Click Invite to retry, or try a Link & Code invitation.

That means: 
Your customer has a restricted number or a spam filtering system that is blocking your invitation.

What to do:
If your customer can easily adjust the filter that blocked the invitation, then you can click the Invite button to send another invitation OR use the "Link & Code" invitation method workaround.