Sounds like a codec or transmittion coverage issues. I am in an area between mobile towers, sometime when the connection hand over from one tower to another poorly, my phone sounds funny. Is there any maintenance work on the mobile network in your area? Installed any new app or ran out of space on the phone?
Are you using an iPhone 7 like the OP, mattybrian?
I have an iPhone 10 and my friend in the same building with the same problem is using a Meizu 16S (android 9).
Optus said that we are in a full signal area (we are in the North Sydney area so its very close to Sydney) but we are possibly in a black hole between two towers.
We live in an apartment block, living on the top 8th floor, with lots of windows. We both had a Telstra phone prior to this and didn't have any issues. The building is concrete with lots of wide windows and balconies.
That does sound like could be the reason. Inserting a Telstra SIM in the phone(s) and testing it out would probably give the conclusion.
Yes we have done that and Telstra does work fine in these phones in this location.
Is there anything Optus can do in this case or are we stuck with a bad connection here?
To resolve the network issue, probably not, at least in the short term.
The "possibly" in a black hole response was interesting, it should be fairly clear cut on their network status map that you either are or aren't.
You can contact Optus directly to see if you call under the Optus Coverage Commitment (there appear to be bugs on the website which I've reported but here is the image):