This is exactly my situation -- sadly still in 2022, 3 years later.
I purchased PUSH which is from what I'm reading and experiencing, not robust and prone to instability.
With PUSH on and working then yes it does exactly what it's designed to do.
With PUSH on it is also prone to "just" dropping registration with no warning and sitting unregistered and missing calls for hours or even days. Only resolution that I've noticed is the cycle the network or app, or phone. It never seems to come back on its own or when I directly try to push re-registration.
So I'm seriously considering dropping Zoiper's PUSH service altogether because why bother if it's so inconsistent and causes constant uncertainty?