I don't use Tomato. If the router looks at IP precedence or DSCP bits of the packets then you don't have to reclassify the packets and allocate bandwidth. I would start with enabling QOS and do lots of downloads/uploads ( you can upload photos to google photos, download apps) and see if the voice quality changes. If voice quality is good then don't do anything else.

If it does, then you can reclassify the packets based of source MAC address (or IP address if router does not provide that) and allocate 170 bps (for three way calling, about 85 for each call) for the Voipo adapter.