While one of the coolest features of the podcasts is the ability to time shift. I can record this on 4/2 and you can listen to it years later. There is one element missing in this and this is live feedback. You get feedback, but it is later when your audience consumes it. Like most things, the more features you add to a product, the more complex it gets. Taking (and recording) phone calls is one of those things that its hard to do without adding more mixers, cables, etc.
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First Words of Wisdom: Don't Do This
Unless you REALLY want to do this, don't. You will need to (more than likely) pick a time when your audience knows you will be there. Keep in mind when you first start out, you DON'T HAVE AN AUDIENCE YET. So you will be adding extra complexities that will burn you out as a new podcaster and make you want to quit.
How The Stars Take Phone Calls on Their Podcast: You Call Them
I know Jillian Michaels will announce on her Facebook page when she will be in the studio recording her calls. If you are free during that time you send her staff and email with your name and your question. They call you, and get you on the show. I believe Ice T does the same thing.
A Free Way To Take Calls: Google Voice
You can get a free phone number from Google (better known as Google Voice) and route that number to your smart phone (thus saving you from giving out your cell phone to the public). You then talk into your microphone and your phone at the same time. The microphone sends your voice into your mixer, and the phone sends your voice to the caller. To get the caller into your mixer,  you can use an 1/8" to a dual  1/4" cable (like this one - affiliate link). You then send the mix of you and your phone into your computer or portable recorder. The Good News: It's free and its easy to control. The Bad News: There is no cue (a line of callers visible to you). Also, as this is your cell phone your wife may call into the show. Reality: Not a bad option. The lack of cue is not an issue as you may not have any callers at first.
Skype in Numbers: $60 a Year
It's very hard to find pricing on the skype site. A forum post showed the price as $60 a year. Here again, you have no call "cue" and it is first come first serve. If someone has called in, the other callers will just here the phone ringing and ringing. The Good News: If you have a co-host this is an easy way to get everyone to hear everyone The Bad News: Adjust sound levels is very limited. You will more than likely become acquainted with The Levelator software (free) and Auphonic.com
Call in Studio: An Inexpensive Call Cue Callinstudio.com charges .03 cents per minute for every caller (you can limit the number of people who can call in at the same time). The phone number is $6 a month. My most expensive show was around $8.00 for the episode (meaning $2 worth of callers). If you do an hour long show it will cost you $1.80 (.03 * 60 minutes).  In a nutshell it would take a LOT of calls to get close to the $40 that Blog Talk Radio charges. Connecting you to the caller. Here again, you can use you smart phone to call into the callinstudiocom. You can use Skype (free) to call into Callinstudio.com. You then take that sound and send it into your mixer. From your mixer into a recording device. You may have issues if you only have one computer. Sending sound (the caller) out of a computer and back into the same computer to record is the definition of feedback. You are feeding the sound of the caller back into the computer. This is where you need a mix minus. This means you send the sound of the mix (you and the caller) back into the computer minus the sound of the caller (this is where a portable recorder comes in nicely - you can have the main out go to the recorder and use an auxialry send to send the sound of just you to the caller.
The Good News: You have a call cue. You can have a screener. You can call guest.
The Bad News: You need an auxiliary se