This should be supported as the conversation is paused and no data should be passed through Rasa while the user is on the phone with the agent. You can trigger this event through a custom action by returning a ConversationPaused() event.
As this conversation will take place outside of Rasa, the choice of VOIP provider is up to you. Is it always voice or does the user have the option to text? Without knowing much about your use case, I’d imagine some users have chosen to text because they don’t want to talk on the phone. So, I’d ask the user their preference before switching to voice and offer both as options.
As far as channels, the Twilio and WhatsApp channels should support this functionality.
Here’s a couple other threads on this topic: