when clicking on carriers tab
When you go a carrier page, the default tab is "Info & Phones". There, you can filter it to smartphones, by checking the OS(es) you're interested in. So if you check Android, iOS, BlackBerry, and Windows Phone, you'll get all smartphones.
But I suppose you want to filter to see just feature phones? Because you don't want a phone that requires a data plan?
You can now search for feature phones in the Phone Finder.