why does my new 13 pro say "Last line no longer available Do you want to call using your remaining line?"

Did you have a previous device that had two lines on it. Contacts on those devices are set to a particular line in a two line phone. I'm not sure how the phone decides how to identify that line, if it has to do with just the number of if it has to do with an ID unique to the phone. Did you tell it yes about using the remaining line?

I've seen this on my 13 pro several times as well.

I've never had more than 1 line.

I just select to use the remaining/available line when I see that prompt and it works fine.

I didn't pay too much attention to it, but I believe every time I saw it was the first time I had called that contact with the new phone.

Funny, I never experienced it on my device, and I've had each of the two line phones, but only with one line. I had read where there is a way to determine what line to use for specific contacts, and that is supposed to be setup in Contacts. I just thought that you could select to use the remaining line, but the OP did not respond to that.

Like I said to the other poster, I'm not sure either, but it seems that it does not happen to everyone. If you notice that it happens to a select number of contacts, you can always delete those particular contacts and then add them back again. But if it seems to be random, I would just choose the remaining line and let it go as nothing more than an inconvenience.

My wife had the same problem with her new iPhone 12. (New to her this last gift season). She tried calling some relatives and received the message "Last Line no Longer available...".

(1) In Contacts go to the particular contact involved. Select Edit (upper right corner of screen).

(2) On "Preferred Line" press arrow right. Preferred Line screen opens. Select Primary. ( The choices are Last Number or Primary. Last Number was checked. Check Primary to change to Primary.) Press Done (upper right corner). Screen for preferred Line closes.

(3) Now Press Done in the contact information screen.

This is clearing up the problem for my wife.