I signed up for FNB Talk yesterday and configured my Telkom Mega200VWR router (Rebranded Billion) with FNB connect's SIP server details. It worked perfectly and I could hook up a standard POTS telephone to my router and receive and make calls on my FNB provided 087 number. Just use the settings:
Server: voice.fnbconnect.co.za
Username: <your 087 number>
Password: <your password provided via SMS>
Clear proxy/domain settings.
It seems to Just Work, but I couldn't dial my 087 number from my Telkom landline, I will investigate it further and contact FNB about it. Dialing out to any number and dialing in from a cell worked.
I am currently trying to reverse their PC "Digital Phone" client to figure out how it sends SMSes using your account. It's coded in obfuscated Java and uses SSL to connect to voice.fnbconnect.co.za. Anybody also interested in figuring it out, please PM me.