1. You can only have one device connected at a time. So you could have either the desktop app, mobile app, or an ATA/3rd party device; connected at one time. If you try and connect more they will just keep stealing the registration from each other and you won't really know which will ring if any.
2. Yes, they are free - no charges what so ever. Not all of Canada is free, but a lot of the major places are. To give a quick example - if you live in Ottawa you can call Vancouver for free. As you say, bandwidth charges apply however you are connected and there are far too many variables for me to give examples

It is always free between Fongo users. There are places that are not free which include some places in Canada, and the rest of the world. The free calling cities are always free though.
3. This depends on if you go with Fongo, or Freephoneline. They are basically the same service behind the scenes, but there are differences in the setting up and running of things.
Fongo - Has Home Phone. There is a one time set up fee of $59 and $14.95 shipping. Then a monthly charge of $4.95 a month. You can also choose to bring your existing number with you for $20. What you will end up with is an ATA (a box that connects to your internet and then you plug a phone into it). It will basically act as a normal house phone from Bell, apart from you won't have the high monthly charges and you get to call a lot more people for free. There is support with this service.
FPL - has an Unlock Key. This basically provides you with details that allows you to connected your own device to their system, and enjoy the same service as with Home Phone. The Unlock Key costs $99.95. What you would then do is buy an ATA (usually around $50) and then set it up using your details. You will then connect it in the same way as the Home Phone ATA and it will work the same. There is NO monthly charge for this service, just a one time initial fee for the unlock key. There is NO official support for this. If something doesn't work you have to either work it out yourself, or ask here or other forums for help. Some would argue that we here are better than official support anyway

You are more on your own doing it this way but you save the $4.95 a month. If you are not technical or willing to roll up your sleeves then Home Phone might be a better way to go.
4. a) You will get a real free Canadian number in a city you choose when you sign up.
b) Either service can be used completely for free without you having to pay for anything (apart from bandwidth). No upfront or running costs. You can use the free mobile app with Fongo, or the desktop app with FPL. You will only need to buy long distance vouchers if the number you want to call isn't free, or you want to call another country.
I'm sure all this has made more questions than answers, so feel free to ask away - someone will most likely answer.
