rule or restriction for using the pushkit framework and sending only notifications without using the voip calls?

I am looking for a simliar answer as well. The app in quesiton has both VoIP and 'regular' aps pushes (alert, badge, etc.) but the documentation doesn't explicitly say one way or other. Did you find a definitive answer?

Yes. As far as I know, Apple is very clear on the usage of Voip Certificates/PushKit for Voip Apps. If your Voip app contains things other than voip calls etc, Apple may reject your app if your using only Voip Certificate for non-voip functionalities as well. It's always better to be ready with both the non-voip as well as voip options in your app as well as server so that anytime you can switch to a combination of non-voip & voip in case Apple objects.

One more thing, it will also depend on the kind of functionality you want to achieve. (e.g. Voip Pushes are delivered straight to the app without user intervention, however, standard pushes need user permission. Also, Voip Pushes doesn't support alert/badge/sound etc as part of remote notification. For Voip pushes, you may need to implement local notifications for showing alert/sound etc.)