When you enter a vehicle as passenger whilst using SPECIAL_ACTION_USECELLPHONE the special action will break, though GetPlayerSpecialAction isn't updated and will stay as SPECIAL_ACTION_USECELLPHONE.
Here is an example of how to prevent entering a vehicle as passenger whilst using SPECIAL_ACTION_USECELLPHONE from breaking the special action if 1) you successfully enter the vehicle then leave it, or 2) you start entering the vehicle but walk away (cancelling the entering vehicle action) .
https://pastebin.com/9Nv2knLu
Here is an example of how to prevent entering a vehicle as passenger whilst using SPECIAL_ACTION_USECELLPHONE from breaking the special action if 1) you successfully enter the vehicle then leave it, or 2) you start entering the vehicle but walk away (cancelling the entering vehicle action) .
https://pastebin.com/9Nv2knLu