Miles: If they hear you well your outbound audio path is ok, meaning that you should be downloading something heavily during your voip conversation or they have problems with the upload at they’re side, this is easy to check.
First ask your other party to ping your destination router to see the amount of packet loss that they have present, if they have good result, the problem is obviously something in your network downloading heavy traffic, if they have packet loss then is an issue at the side of your second party.
Regards