Gocall's phone app is a modified version of groundwire by Acrobits. Unfortunately you can not select external sip providers or logon to gocall without a account, but if you load it up, you are able to make voip calls on it for free, but people can not call you back. There is no acceptable free voip application on the iphone, this is the next best thing.
I use it mainly to call into talkshoe via their sip number [email protected] it works very well.
Try it out.