First, check to see if the user has their global availability set to "Away". If they are not set to "Available", calls will automatically be sent to their failover address. This can be checked in the following ways:
- Have the user log into the OnSIP app and click their profile picture in the top right.
- From another user's OnSIP app, view the first user's contact page.
- View the user's Personal page.
- In the Admin portal, go to the Users tab and view the Status column for that user.
If this is not the issue, it is typically a NAT problem.
The phones open a NAT port and then wait for an inbound call. We send a call to that port, but the NAT has closed the port already.
To fix this, OnSIP sends a keep-alive when we see you are behind a NAT. However, in your case, we cannot detect the NAT because your firewall is replacing your internal IP address, e.g. 192.168.0.105 with the external IP address.
If you can get your firewall to stop any "SIP Translations" or "Application Level Gateway" it should resolve the problem.
You will know it is corrected, when you look at your Registration Details and see that the "Contact" IP is your internal address and the "NAT Address" is present.