Facetime audio via landline, why no good warning?

Why isn't there a better warning regarding using Facetime and mistakenly choosing a landline telephone number? We made that mistake today and are now trying to negotiate away the $135 charge that will appear on our telephone bill. The particulars are given below:

My wife has a cousin who lives in Lima, Peru. She has had Facetime sessions with her cousin in the past, both video and audio, using the Internet. These sessions took place using either my wife's iPhone or her Macbook Air laptop. This time my wife mistakenly chose her cousin's landline number, in a Facetime audio session using her Macbook Air. The landline number and several mobile numbers presented came from her Contacts, which I presume is shared between her laptop and her iPhone. A message popped up, saying that to proceed it was necessary to arrange to have her iPhone connected to the same wifi network as the laptop. Had we had our wits about us, we would have stopped there. As it was, we turned on the iPhone's wifi, and the Facetime audio conversation then went on for 45 minutes. When the session ended we received a text message from T-Mobile, saying "Caution! As of 03/24/2018 your T-Mobile account reflects 135.00 of talk charges in this bill cycle . . .". By then the "horse was out of the barn".

It seems to me--in this circumstance--we could have been warned about possible excessive telephone charges--at the same time that the message told us to have her iPhone connected to the same wifi network.