If you are having issues with audio on push notifications, you should follow the same troubleshooting steps for issues with audio on any calls.
"The call connects, but I can't hear the other party and they can't hear me."
"The call connects and the other party can hear me, but I can't hear them."
"The call connects and I can hear the other party, but they can't hear me."
In addition, there is one push option you should try adjusting.
Tap on settings, then SIP Accounts, then your SIP Account. Tap on incoming calls, then push options. If Simulate NAT is on, turn it off. If it is off, turn it on. If this doesn't solve the issue, set Simulate NAT to on. This is what it will need to be for most providers (One exception, 1und1 users should always have Simulate NAT off).
If that has no effect on the issue, just follow the steps in the applicable article above. Be sure to note that the issue is specific to Push Notifications if you submit a ticket.