"I can receive calls when the softphone is open, but not through Push Notifications".
Try the following.
1. Tap on settings, then SIP Accounts, then your SIP Account. Tap on incoming calls, then push options. Turn Single Registration on, then save the change.
2. If you still have an issue receiving push notifications, go back to push options. Turn Simulate NAT off. Save the change and try again. If this doesn't solve the issue, be sure to turn Simulate NAT back on.
3. If you still have an issue, you can submit a ticket. In the ticket, note that you have you have gone through this article and include your username and domain so we can check if you are registering for push notifications.
Note for 1und1 users: The settings for 1und1 require that Single Restration be on and Simulate NAT be off. But even with these settings, 1und1 still has severe restrictions regarding multiple registrations. If you ever have trouble registering (either when the softphone or Groundwire is open or after accepting a push notification), close the softphone for about an hour or so and you should be able to register again.