You cannot know the carrier because you can keep your number when changing them. I'm with Vodafone and have a 0176 dialing code carried over from my previous co

You cannot know the carrier because you can keep your number when changing them. I'm with Vodafone and have a 0176 dialing code carried over from my previous contract. But as others pointed out, in the US mobile numbers get normal landline dialing codes so you cannot see a difference. So, in a way, yes, it's possible for some customers to automatically detect that, but then you need to question how many of them come from countries where there is a difference and whether it's really a good use of your time and money to implement something that might only benefit a percent of your users. As for me, even though I'm conscious of UX, I wouldn't care much if I had to classify my phone number once. What they described in the article already goes way beyond what most other signup forms do. (Fun fact regarding area codes and figuring out a city from that: The village I lived in previously was pretty much halfway between two smaller cities and it had the area code from one but administratively belonged to the other. So not even that is easy ;-))