There is no sound from the handset in the original firmware or in Openwrt. The microphone goes, the other party can hear me, but I can't hear her. I have a TP-Link MR 200 v1.1 O2 router, but it appears to be v1.
I tried the instructions and installed viz.:
Thanks for the tip, I'll try it. It works if I connect the mobile data via wifi hostpot and do wwan in openWRT. It also works on another Netgear WNDR 3700 v1 router, which I checked has the same settings. So there is a problem somewhere between the transmission between OpenWRT and part of the system from TP-Link, the author writes "The included modem in this device is running Android OS and has a hidden" WAN usb0 and WAN6 usb0.
There was a problem even with the original firmware, where it was not possible to operate even according to the TP-Link instructions, and I thought that OpenWRT would allow it.