Slight dilemma....

Google Voice gives you a phone number that you can use to make and receive calls from the desktop browser, and if you want, you can set-up Google Voice to forward the call to your phone (any phone). With that said, if you do forward Google Voice to your cell phone, then yes it will use your cell minutes (so no, it is not free)

other options, depending on where your dad works, his company might offer employee discounts with your cell phone provider (FAN Discount). I get 20% off of my voice plan from when I used to work at Home Depot, even though I quite a long time ago, the discount has not been removed from my account, so my bill is around $60-$65 a month (450 minutes/200 text/Unlimited Data)

 

How many minutes does he use in a typical month? If he has Sprint coverage in the area consider looking into Virgin Mobile. Their $25 plan for 300 minutes and unlimited text/data sounds awfully tempting, that is if you're willing to make do without iPhone.

 

Well, let's ask a more fundamental question here: is your dad really going to use an iPhone 4? How exactly will it benefit him?

I learned a long time ago that pushing technology on people who don't want it on their own, hardly ever has a good outcome.

My dad, for instance is 50. Not that old, and in fact he knows about computers quite well and is very technically inclined. He enthusiastically bought me my first computer, and even enjoyed playing video games with me on it. Even today he takes advantage of MMS messaging on his recently-aqcuired Sprint camera phone, and subscribes to satellite radio.

But does he have a computer? Nope. Nor does he use the internet, which the house is connected to but is used entirely and paid for by a sibling who lives there. And frankly, he doesn't want it, no matter how much I or my siblings or other relatives have suggested he get one.

We've tried getting tech stuff for him before for years, so we know very well what'll happen if we get him an iPhone or a computer: He'll barely use it, or not use any of the apps. Facebook, Twitter? Forget it! He'll likely get frustrated by the virtual keyboard and the texting he does will probably dwindle as a result.

This is a pattern I've found with a lot of people, mainly older generations, who have technology foisted on them. If they don't seek it out, if it's not inherently interesting to them, they won't try to learn about it and just get frustrated with it.

Don't get me wrong: my dad sees my iPhone, and sees my MacBook Pro when I fly in to visit for the holidays, and he's in awe over this stuff and appreciates what it does, even notes that these things look way better the things they show on "those Windows commercials."

He even got a little depressed when he accidentally broke his basic camera phone that he had all his pictures stored in, and was a little relieved when I was able to partially restore his photo collection on his new phone by MMS'ing him everything I had on my Mobile Me gallery to him through my iPhone. He then appreciated the concept of Cloud Computing and how it could've helped him.

But he puts his foot down and will not get an iPhone, iPad, Android device, or a computer, nor are any of us allowed to get him one. He just doesn't want it. He recognizes is it's a big part of my life (in fact it's my job), and that of the rest of the family, but it's just not for him. He doesn't see the benefits as being big enough for him to want to learn any of it, even though for most of us, iOS is dead simple to figure out.

So we've kinda resigned ourselves to the fact we have a luddite dad of sorts. He gets along through life just fine, and so we find it kinda neat that he's proof it's still possible to live life without actually being online.

We don't have the heart to tell him his phone, although not a smartphone, still has a WAP browser built in and he can install Opera Mini on it...

TL;DR: You may want to consider your dad's reasons for resisting. Having an iPhone pushed on him before he figures out it's a Good Thing on his own might get him frustrated with it and actually turn him off to using it.

 

Scaredpoet has a good point. As long as he's ok with technology and uses it I think he will enjoy it.

About the bill you can look at ATT's A List service which allows you to hace 5 or ten numbers that you can have unlimited calling with. You could get the google voice number then just put the google voice number in the A List numbers so that all of his incoming calls at least are free.

 

You being a fanboy is making this difficult. It seems like a pain in the ass , not to mention it is more costly for him to have an iPhone by your own admission.
Why cant you just let your Dad do what works for him? Is it that important that you are satisfied that the fanboy in you talks him into getting something that is complicated more costly and a device he probably doesnt need?
If he wants the technology then great but there are other phones that are cheaper for him than an iPhone and apparently that matters.