1. Smart Link Backup means that only one WAN connection is used at a time. If the primary fails, than the secondary will come active. If you want to use the 2 WAN connections at the same you have to chose the option Load Balancing. Make sure to put correct data in the field Max Bandwidth provided by ISP. And for the Network Service Detection, whatever IP you chose make sure that it replies to ping and will not give false positive for WAN failure. Most probably at this point the router use the second WAN, exactly because of that.
2. You can configure that IP 192.168.177.9 use only WAN 2. This is done by the menu Protocol Binding (it is active only when Load Balancing is used) For service you chose Any, Source - is the IP of the phone, Destination - 0.0.0.0
Hereby the admin guide with more details about the options:
So, you can hear the phone ringing and answer it? which means that SIP pakets are coming through WAN to LAN and well redirected to the phone IP, but you cannot hear after that, which means that there could be a problem with the RTP packets.
If you have problem only with the incoming calls and not the outgoing, than try enable/disable SIP ALG (Firewall). If that doesn't fix the issue, try to allow (or even forward) from WAN to LAN RDP - UDP ports 16384-32767 to the phone IP.
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