I have a MI424WR-GEN2 router, and an ESI IP phone like this one:

I have the phone set up in my home office (in DE) that links back to the main office (in VA), and am having a lot of stuttering and skipping in the audio on my end-- but calls sound fine to people on the other end. My FiOS connection is a stable 25/25, according to SpeedTest.
A bit of research shows that the remedy is likely enabling either QoS or port forwarding, but I can't seem to find a straight answer about which it is, how to do it, or what settings to use. Can anyone help me get this squared away?
Thanks!
JS