Acrobits Softphone[1] and Groundwire[2] support iOS push notifications. I have found they work much better than Bria[3].
[1]: https://apps.apple.com/us/app/acrobits-softphone/id314192799
[2]: https://apps.apple.com/us/app/acrobits-groundwire/id37850308...
[3]: https://apps.apple.com/us/app/bria-mobile-voip-softphone/id1...