In my series of cheap carriers I just tried out Tello on an iPhone7 and a Pixel. The signup process was really easy and I was able to port number from Sprint without issues. Sprint makes porting out very simple, the PIN you use for the account is the port out PIN, you also have to find the account number on a bill that you can download as a PDF. Give this info to Tello and wait one night, they will email you if it worked. The email has documentation links for the codes that will setup your iPhone ##25327# or Andriod Pixel *#*#72786#*#*, but do use those links as I oversimplified it, in general they want you to be on WiFi for iPhone and some other bits. On iPhone the setup was pretty fast and painless, on Android it took a while for the setup to complete, also it kept doing setup after a reboot. It takes so long that I cancled it a few times.
They have a $10 referral code available to sign up. Both the current user as well as the new signup get it.
