Care to point me at one that will allow me to SMS using my VOIP account?
That's the very point I was trying to make.
You need to look at your VoIP account like a telephone line, not like your mobile phone !
VoIP is the SIP protocol. Hence all the softphones out there will be doing SIP.
It really shouldn't come as any surprise that there are no softphones that do SMS, because there is no demand.
SIP is an open standard. SMS would require interactivity with supplier APIs which would mean headaches for softphone developers !
So as I said, if you want to do SMS, you'll need to use different software or a website.