Hi bcsaba.
You are right about sx763os working on administrator acc, just checked that and it works.
The reason why administrator password is not accepted is because there is a ssh public key located in /config/certs/pubkeys/administrator/ named 1copy1.
If any file is located at /config/certs/pubkeys/
username/ than ssh server uses that for auth, and it ignores one from /config/system_configfile_r.
I assume that public key is only located in t-com and other branded fw-s, so by loading some older fw, which doesn't have that public key, it will accept web password and than we can get shell access.
About the voip on openwrt, it works ,even without asterix, ( check
http://code.google.com/p/danube-voip/ ).