In order to prove your identity AT&T sends an text message to your phone. This is not helpful if you have already lost your phone. You can have that password sent to another phone on your same billing account. Once your account is created, you can use AT&T’s network to locate your lost phone. If the phone has a GPS and that information is shared with the network then the location can be known to a few feet. Without GPS it will give you a rough estimate.
Family map is free for 30 days if you cancel during the trail period. It does not offer the other features that iCloud has such as remotely wiping or putting a message up on the screen but it is a good alternative if you have already lost your phone.