iPhone 5 tty symbol randomly appears

My mom has an iPhone 5 - we recently got her phone unlocked and switched to a different carrier. BUT this also happened before we switched carriers or unlocked. The TTY symbol randomly appears on the top right by her battery.

***PLEASE NOTE*** The most popular "solution" is to go to Settings > Phone > TTY (turn off). I DO NOT have this option. Are there any other solutions? Thank you!