I've used a couple of SIP clients on the iPhone, iSIP (formerly SipPhone) and Siphon, available via Cydia for jailbroken iPhones. I also use 3GUnrestrictor, so I can do VoiP over the 3G network. With the iPhone, I have an unlimited data plan, so I don't worry about the minutes, but I'm usually on wi-fi when I use it anyway. On 3G, quality is inconsistent, and varies from time to time, and place to place. Usually, I'll do a bandwidth check first to ensure that I have adequate bandwith before using the #G network for VoIP.

Fring was the first VoIP client I tried on the iPhone. Although it worked well (I used it on a trip to France to call back home) it has one major drawback. No access to the keypad in a call, so you can't access voicemail. Also, like Siphon, you can only configure for a single SIP account. Since I have several SIP accounts, iSIP is ideal for my needs.