That's a very strange protocol combination for a smart plug to use. SS7 and IPDC are—as you already point out yourself—protocols for VoIP communication. I can't say that I've come across those two in particular in the IoT environment. There might be one reasoning behind that however. Do keep in mind that the following is just a guess one that makes certain sense. But I can't offer any proof for this.
Since smart plugs don't care about any of the usual low power considerations of IoT protocols since they are mains powered they might choose their protocols for other reasons. One reason that seems to be likely when VoIP protocols are used (besides arbitrary picking by the developer because (s)he knows it) is quality of service. Almost any router will prioritize via QoS VoIP packages. That reduces latency and increases reliability for the application on top. Maybe they tried to game that system with the plug.