I had some issues with my Gigaset Go Box 100 (it's a VoIP DECT station) connected to an OpenWRT router (it's a TP-Link Archer C2600 with the latest 18.06.4 firmware).
I can't establish phone connections to some numbers of my callers. Sometimes I can't even here a free line signal on some phone numbers. And sometimes I can hear the ring signal and when the call is answered the connection can't establish.
If I quickly change my network to a router from my provider and connect there the Gigaset Go Box 100 it just works.
In the manual of the Gigaset it's described that I could change the SIP and RTP ports of the device and test it and furthermore if it's still not working to enable port forwarding to the Gigaset device.
Now I forwarded the ports 5060-5074 UDP for the SIP connection to my local Gigaset device and also the ports 5004-5020 for the RTP streams.
Now it seems to work. But I wonder why this works?
Normally the connection is initiated by the Gigaset device and I should not need to forward the ports. Also without the forwarding the Gigaset box is able to connect to my ISP and I can phone some (not all!) numbers.
I read that's not common to port forward all ports to the voip device. Especially I'm planning to use a second voip device and here I can't port forward the same standard ports to my provider (which seems just to accept the port 5060 for the SIP connection).
Is someone using voip from Deutsche Telekom AG (Magenta TV) in combination with an OpenWRT router?
Thanks in advance!