Hi Dennis,
A couple of points:
1. This is beginning to sound like a classic firewall problem, with the ports closing after a while of inactivity. You should really check whether you have properly forwarded the ports of the W60B on the firewall/router/modem that sits in front of it. It's kind of an important requirement for STUN devices with regards to reliability and should be your first go-to.
2. The latest 3CX supported firmware is 77.83.0.20 - Ensure please that it is running this version. You can get it from
here if needed. You must click "Software" on their page to see it.
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