I use a commercial "better than G729" codec for continuous 2-way voip over UMTS. G729 "closes" an up or down channel if no-one is talking on it. With G729, when a non-talker starts to talk on an overloaded network, there is sometimes a delay before he can be heard clearly again.
Out-going voip calls are easy. In-coming, always-registered, voip calls require a bit more work to set up correctly. Understandably, some networks make more effort to try and stop external voip callers from dialling in.
Over e+, I need 4MB for 10 mins speech. At 24c/MB, long, out-going conversations may be cheaper via a mobile. For longer voip conversations, a flatrate data tariff may be more suitable. If you have nothing better than G729, I would stay clear of e+ because they sometimes offer gprs speeds instead of UMTS. If you only have G711, don´t waste your time trying e+.
Inquisitor has previously linked to a page that reports UMTS voip is possible with a Tchibo sim:
Germany - Tchibo - Prepaid Wireless Internet Access
Tchibo uses the faster (than e+) O2 network. Tchibo has a cheap prepaid monthly data tariff. It´s something like 10 euros for 500MB/month.