Getting a contact by Whatsapp and not being able to call the number without saving it to your contacts.
-Someone sends you a number via whatsapp you cant press on it and make a regular phone call, only a whatsapp call
Not the most ideal but in Android, you can tap to message the person, then tap their number on the top to view their profile, then scroll down and hold on their number for 2 seconds and it gets copied, them open dialer and paste in the dialer.
I'm sure there's a way to do the same on iPhone.