Credits to Val, thank you for your help.
The attached PDF will help you set up your Grandstream HT702 device from scratch. It is a known working configuration so if you are having troubles with your Grandstream HT702 reset back to defaults and follow this guide.
Edit: Set “SIP Registration Failure Retry Wait Time” to 120 seconds.
Disable "Allow Incoming SIP Messages from SIP Proxy Only". Change that to "No". FPL to FPL and Fongo to FPL calls may not work with that setting enabled. Fongo Home Phone has that setting disabled.
Do what Mango wrote below.
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