I have seen the request in different forms. I too stumbled on bandwith control for guests and Voip traffic. A clean and universal solution could be to prioritize VLANs. VLANs are supported throughout the Peplink range, so you could make a Voip VLAN with high priority and a guest VLAN with low priority. And for the more top-of-the range models even a numbered priority scheme.
An added bonus for a Voip VLAN would be that you don’t have to check the protocol/port (application), so that a new type or unknown devices can be prioritized by putting them in the correct VLAN.