Or you can use FaceTime for audio/video "calls". But you can't make convential phone calls like a phone.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
How to Block Someone on FaceTime
My Facetime Doesn't Ring
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.
The best app I found is Fongo. Available in the Canadian appstore only.
Free with an ads banner. It gives a free phone number. It allows 911 calls. You can receive and make free calls to most of Canada. You can receive free sms from anywhere and make free messages only to fongo users.
Paid services are available in 4 packages:
1-World Credits: $5, $20 and $50 Credit to make international calls and to canadian cities outside free calling areas;
2-Unlimited Canada Texting: $1.99 for 1 month, $4.99 for 3 months and $9.99 for 6 months of unlimited Canada texting;
3-Unlimited Canada + US Texting: $2.99 for 1 month, $7.99 for 3 months and $15.99 for 6 months of unlimited Canada and US texting;
4-Permanent ads removal fee: $2.99
The second app I recommend is Magicjack.
Free without ads. Free subscription allows free outgoing calls to USA and Canada and incoming calls from MagicJack users. $10 per year to get a US number for incoming calls.
Canadian numbers are avaialable for $20 first year then $10 per year.
No sms services available.
The third best app is Google voice and it alows free calls to USA and Canada. In order to receive calls you must subscribe from USA on http://voice.google.com to get a free US number. Then you can get an app called vtok from appstore to receive calls. This app will not give you notifications of incoming calls unless if it is running.
Vonage is a good app for free calls to US, Canadian calls cost 1 cent per minute (used to be free).
Skype is an interesting app that allows you to subscribe to different services.The best is the 2.95 per month unlimited US and Canada with the following options:
1-A caller ID with your US or Canadian number
2-Skype to Go Numbers (local numbers assigned to skype contacts and long distance phone numbers)
Another service is the Skype Numbers (phone-to-skype) available in 2 packages:
1-3 month subscription of $18. Sometimes 33% off
2-12 month subscription of $60. Sometimes 50% off