The biggest advantage of using VoIP on iPhone is the money saving when making cheap long distance calls. For example, you can call the US from anywhere in the world for as low as 2 cents per minute using iSip or Skype for iPhone. You can even make completely iPhone free calls to the USA and Canada using NetTALK or iCall.
You can make really cheap or low cost international calls with VoIP. Moreover, you will be able to save some of your monthly plan minutes when making your calls over the internet.
This is one of the reasons why cell phone carriers don't like VoIP on iPhone and blocked the VoIP feature on the iPhone for so long time. It is also why Skype might start charging for Skype calls made over 3G. The iPhone VoIP is always free of course over wifi.
Disadvantage of iPhone VoIP
VoIP calls with your iPhone are really good but they will not replace your regular cell phone monthly plan for simple reasons.
First, you won't have a dedicated phone number unless you pay extra for a SIP account with a phone number.
With most VoIP iPhone applications people won't be able to reach you by dialing a phone number.
Generally, your party must have the same application running on their computer as the one running on your iPhone.
For example, if you want to use Yahoo messenger, your party must install Yahoo messenger for Mac or PC to establish a connection with your iPhone.
If you want to make a skype call, you must have Skype running on your iPhone and they must have Skype running on their computer and so on. Moreover, their Mac or PC must be on and they must be logged in.
Another reason why VOIP iPhone calls won't replace your regular cell phone call is that the sound quality might suffer sometimes depending on the network speed, the iPhone network, and the area where you are. This reason makes voip phone calls not reliable in general, so you should expect call drop and voice lagging and echo sometimes.
you can see, you can basically save your minutes. Therefore,
Apple and AT&T blocked VoIP access over 3G/Edge network for more than 2 years on the the iPhone after it was released. Finally, they removed this restriction.