It would also help to do a wired speed test from the remote end of the 7731 P2P bridge via a laptop.
Because there isn't really much / any buffering action going on the P2P bridge, you will find that your internet speed can be worse than the bridge's maximum speed, depending on how your ISP's cap is implemented (e.g. is it throttling or a medium limit?). You're traveling through two bandwidth-limited links (the 7731 P2P bridge and then your internet connection)
Also, throughput testing might not be the right mechanism. VOIP is latency sensitive too, so it might be worth testing both the latency across the bridge and also the latency end-to-end to see if that's a contributing factor.