honestly, i personally havent tried using any of those services with my $19.99 plan. I can tell you now that i pretty much been able to do anything else i have wanted to do. FTP, streaming audio/video, filesharing, etc.
maybe when i get home tonight ill try out some VoIP solutions for you and see what i can find but to be honest i really dont think you'll have a problem.
in all honesty though being able to use the $19.99 MediaNET plan with your smartphone is just a glitch in the system. i honestly dont think they realized that people would be using there smartphones to teather to laptops. the whole reason why they have the PDA connect add-on ($44.99 w/voice plan) is because they encourage those individuals to teather and use all the data they can either by browsing or downloading emails, etc.
the average user usually wont spend $400USD+ for a phone, mostly because the average joe can't afford it. if you cant afford or wont buy a $400USD+ phone why would you spend $40-50USD a month on internet access for it?
all PDA connect is really is MediaNET in a nice big pretty bow that cost a lot more