The application: a home setup with the Linksys SPA2102 connected to my TP-Link ADSL2+ modem. A VoIP service by Teksavvy. I bought the Linksys SPA2102 by myself, but I didn't know only VoIP devices sold by Teksavvy are supported. I setup the SIP server, username and password in the PSA2102.
The problem: When dialing out (outbound), there was no ring tone on the phone. The call was established but for the first 40 seconds the call quality was terrible. And the call was only half-duplex (out from the Linksys device, but extremely poor quality). After the first 40 seconds the call quality was fine (duplex).
What I tried: I tried different audio codecs. I tried to enable NAT. I tried disabling the firewall in the modem. I tried to enable streaming on the Linksys device. I set to establish/accept unregistered calls. These settings didn't work.
No solution. I'll buy the phone adapter from Teksavvy so that I can get support.
(Note after the post: the solution was to put the SPA2102 in the DMZ and to change some firewall settings - can't get into the details of this last one... )