I successfully made Gtalk, Skype and SIP calls over tethered 3G and edge on my N810 but as someone before said I wouldn't place a business call over that EDGE data connection as its prone to be choppy and unreliable.

What I really wish I could do is get the SideKick prepaid plan which is just a dollar a day for unlimited web and text and then you pay per minute for phone calls. That way when in wifi and 3G I could just use SIP and then if I needed to I could make regular phone calls.