[Q] Strange in call sound between 2 nexus 5s

Im having a really weird issue with the sound quality in call between my gf and me both on nexus 5s. I have had mine for a few weeks, she liked it and picked one up about a week ago. The sound for both of us is hard to describe but it sounds tinny, a high hiss, metallic when one of us are talking, when we're not there is no static, hiss anything, only when talking, almost changing our voices. Neither of us experience this talking to any one else at all. We don't live together so I talk to her regularly and I was thinking somehow it was with one of our towers while at our homes but I have other family that lives blocks from her and on tmo and we don't get this. So we tried in different locations and the same thing so that's out. The weird thing is I can hang up, call her using my google voice number and it is crystal clear, call back using my tmo number and right back to the same sound.

I'm not sure where to go with this. I know I can just use my GV number but it's driving me crazy trying to figure out why the hell it would do this. Any help or insight is appreciated.

Hmm, HD Voice huh. It really doesn't sound soo HD to me. I did a little digging on this and looked up the phones that are capable and both our old phones were. Until we both had a N5 it didn't sound like this. I trust that this is it but i'm not a fan if this is what they call HD. But like you said maybe we'll get used to it.

Thanks for the info!