Step 1: You gotta find the sound path, so go in-game and open the console and type "developer 1" or "snd_show 1" and then let the phone ring.
While doing this look in the console for the sound file path.
Step 2: Recreate that path in "garrysmod/sound/" meaning you put a new sound file in there, same filetype and name which should replace it
from the server, hope so.
Step 3: Put a quieter/silent file there with the same name and it would technically replace your ringing sound with something that
is better in your taste, hence you think its incredibly loud. Give me feedback on if it works, if not ill try myself next week.