Load-balancing of voice and/or video traffic between multiple connections (whether on the LAN or via ISP/internet connections) is NOT recommended. Related packets will arrive out of order due to eventual differences in delay/congestion on one or the other connection(s), leading to packet drops or packet loss. The effects result in poor call-quality experiences (audio cutting in/out, possible dropped calls, etc.), AND inconsistent phone operations.
If there is a need to load-balance your network traffic, it is best to create a policy where all voice and/or video traffic is routed over a single, preferred interface or connection, while all other traffic is load-balanced. This policy should be based on your routers ability to separate the traffic/traffic type. How this is achieved is different on most router brands; please check with the respective vendor for further details.