Hmm… So at this point nymea does not support audio/video streams to be transmitted through the nymea system. For the future we have plans to support that, for use cases like CCTV, doorbell systems etc.
Is your intention to create a proper phone use case? Or you’re looking into SIP to communicate with appliances as doorbells and IP cameras?
In general, a plugin in nymea is free to do whatever it wants. So for instance if you have nymea:core running on a device with speakers/microphone attached, you could very well integrate that with nymea to allow nymea controlling the phone calls (e.g. having an action dialNumber(number) or hangup() etc) but then handling the actual audio/video yourself and routing it to/from speakers/microphone yourself inside the plugin.
Does that answer your question? If you have anything else in mind, please let us know about your use case and we can discuss it.