FWIW, my co-worker has been using CallCentric on his e71/e72, and its been working ok. I had the CallCentric test account (free callcentric-to-callcentric calls) working with an online trunk provider (pbxes.org) and it worked ok...we're both VoIP engineers.

Due to configuration hassles, I opted for "Skype Out" for unlimited outbound calling @ $3/mo., as it allows you to use your current cell number as the outbound caller ID (i.e. everyone you call sees your old cell number). Skype-in can be added for an additional $3/mo. however you are issued a different phone number.

As far as battery drain, I was able to go at least 18hrs logged into Skype all day, without making calls, if not a full 24. This in addition to 3 e-mail accounts polling every hour, OM weather hourly, and whatever the facebook/RSS feed polls at. Any SIP provider is pretty much going to tax your battery as you'll be constantly sending keep-alives, along with some monthly fee. I suggest just buying another battery, either OEM or extended, if you're going to always use the SIP client. I've resorted to just letting calls go to VM, and then logging into Skype to call them back. My total monthly bill on T-Mo is $29/mo for 500 minutes, and $10/mo for data. The extra $3/mo gets me unlimited outbound calling via Skype.