If you use the softphone on the computer, a lot of DellVoice is free.
You will get a free number and be able to make calls to a lot of Canada for free and of course receive calls for free. No strings, no hidden charges.
The only charges you may come across are if you want to bring your existing number (porting in) which is 25$, if you want to use another device other than the mobile app or the softphone you will have to buy the config file which will allow you to use an ATA or any other sip device that works with DV.
Obviously calls to international numbers are extra, as are calls to numbers not covered by the free call areas
http://www.dellvoice.ca/cityListing These calls need a voucher to be bought first. If you try and make a call that is not free you will be told you do not have enough funds to make the call. You won't run up a bill or anything like that if you call a number that is not free.
All in all, if you download the softphone and set up an account, you will be able to make and receive calls without fear of it costing you a bean. Like many here though, you will probably find that you like it enough to buy an ATA so you can use it as your home phone and ditch Bell/Rogers/etc; and their monthly fees.