There are eight Burner Phones that you can find, you get the first one (1) when the Game starts (can't be missed). (2-5) can be found if you unlock all ctOS Towers. But (6-8) are only shown on the Map if you reached Act 3, but you can go there and get them anyway
Unlock: After you collected the 7th recording you unlock the
Vespid LE Car