Using wifi for service?

I'm sure this has been covered numerous times, but just try to search for it and spend an hour rummaging through the hundreds of threads unrelated to the topic... then come back to this thread and scold me.

Having said that...

My mother owns an iphone 4 and is with AT&T. She takes care of our little one a couple hours a week to help my wife and I out while we're at work, shopping, etc.
The problem is that her service (any and all) drops like a rock as soon as she turns down our road. She can't receive phone calls or get messages while at my house. We don't have land line phone service, either. Until now, I've seen no reason to have it.
This is concerning because if she needed to get a hold of us while she was watching the baby, she wouldn't be able to.

The options we're considering is to either purchase a home phone and get a phone plan from our DSL provider (which ironically happens to be ATT), purchase a pay as you go, or leave one of our phones with her. The last option is a rarity. A pay as you go line with our provider (tmobile) is out of the question as their prices (and policy) are ridiculous compared to that of a standard walmart pay as you go. The kicker with my provider is that the minutes actually expire. So, if I buy 30 minutes and my mom only uses 5 of them, that's 25 minutes down the tube... and we'd have to keep restocking minutes at about $10/pop according to the CSR @ the local B&M tmobile. The next best option is 1500 minutes talk for $30/month.
It's not really even the price that bothers me but the fact that I'll have to keep replenishing the supply. Just seems that I can do this easier... but, I digress.

I'm trying to research if there's a way for my mom to use her phone via our wifi network. I'm able to make/send/receive calls/messages on my BlackBerry so figured there's got to be some way to do this with the iphone. However, in the bone stock condition (ie: no jailbreak), it doesn't seem doable.

So, I'm just looking for guidance on if this is even a possibility and if so, what methods can I look in to for my mother to use her phone over our wifi?
Links will be just fine. I'm not having an easy time finding any threads myself, though.

Thanks in advance for any input. If we have to cough up money for a land line or pay as you go phone we will. I'm just trying to see if I have alternatives with my mom's iphone4.

- Erin

 

Thanks, all, for the suggestions.

I'm giving skype a lot of consideration and in my research, I came across the google voice app + talkatone app, which apparently allows the user to make voip calls over the google client. The cost of both apps is... free.
So, next time I'm able to, I'll try to download both apps and see if I can get that to work for us.
In the meantime, does anyone here have any input on this pairing, or other suggestions not already made?

I'll continue my research. I appreciate all the input.