"I don't think it's DD -- she works as a police dispatcher/911. She wouldn't have a job if it was something about her voice."
It wouldn't be the timbre of her voice that causes that, but it might be due to her particular anatomy. Cell phones vary in sound quality due to a lot of factors, and shape/size/microphone positioning can be a big part of it. In her job, she's likely wearing a headset so positioning isn't an issue due to the mic being on a boom. But if her phone is shaped such that on her particular shape of head/face/cheek/hands, she blocks the mic or feedback sensors while holding it, it'll sound bad. The test I usually suggest to see if it's positional is to have her put the phone on a stand or lay it on a table about four inches away from her face, directly in front of her, and speak normally. See if it sounds different from when she's holding it to the side of her head. Also, have her remove any covers or protectors, or try holding it in her other hand in case she's blocking the ports with her fingers. Also, have her call you from a different cell phone (like from your DH's phone) and see if it's any different.
If the sound quality changes due to position, the only real fix is for her to change how she holds the phone, get a headset (a Bluetooth earbud can often fix this sort of problem) or switch to a different phone. If positioning doesn't help, then a chat with her service provider is in order to discuss possible issues with sound quality, since one of the other issues phones can have is related to antenna positioning and broadcast strength.