I did a fair amount of research and thought I should share
I did some research into UMA, from umatoday.com/faq.php
IS UMA BASED ON SIP? WHY OR WHY NOT?
UMA and SIP perform different functions in a mobile network. SIP is an application layer technology that facilitates the creation of new IP-based mobile services. UMA is an access layer technology that enables mobile services (included SIP-based services) to be accessed over broadband/Wi-Fi.
The comments in a howard forums post seem to suggest that UMA is probably tunneling GSM over IP, instead of doing SIP over IP.
Though the specification for UMA (or GAN as it's know inside the 3GPP working groups is available) at http://www.3gpp.org/ftp/Specs/html-info/43318.html
this asterisk 2005-August dev post, seems to suggest that there's a lot of supporting software that would need to be running on the handset. (And unfortunately some of the specs aren't easily available)
Probably a better solution for the E-Series phones is something like gizmoproject.com or http://www.fring.com/
In my experience there a couple of really serious problems with VoIP services. First it can be fairly complex to configure, and second and probably more importantly there are far too many VoIP providers, so it can be giant pain to pick one, and they aren't necessarily all
compatible with each other.
I found one interesting story about gizmo and asterisk creating a global pbx with asterisk which gives an idea of what's possible, though is probably something of a pain to setup.
I have used the built in E61 VoIP application with gizmo and managed to get it to work over both Wi-Fi and EDGE. (Though the latency on edge was pretty bad).