Yes, if you want real-time signalling you're generally going to need a persistent connection, long polling, or at minimum near-continuous regular polling depend

Yes, if you want real-time signalling you're generally going to need a persistent connection, long polling, or at minimum near-continuous regular polling depending on how "real-time" you want to get. For example, if your phone polls Apple/Google for any new push notifications every 3 seconds, does it matter that a Signal call takes 1.5 seconds (on average) to start ringing on the recipient's end? What they're mainly talking about, however, is that it's inappropriate to have a persistent connection (as would be required with SIP) continuously running in an app while the app is doing nothing but waiting for a call. Once that notification comes through and the call is accepted, then of course a persistent connection is required for the duration of the call. When the call is over, the Signal app can shut down all network traffic and wait to be reactivated via a mobile push notification when the next call comes in.