Well, I must confess I was never so naive to try VoIP over eplus' congested last-century network.
I concluded VoIP was blocked, as I remeber some thread on a German forum discussing how to by-pass eplus' VoIP-blockade.
But as you'll certainly agree, eplus is ineligible for such time-critical applications like VoIP.
As to software, I prefer Nokia's inbuilt VoIP-stack, which is superior in terms of integration and quality to anything else I've tested so far. With the SIP-settings tool from Nokia, which I mentioned in
this posting one can manually change the preference of codecs in favor of a low bandwidth one. Unfortunately the Nokia client is only avaiable on Nokia phones.