A pilot may not depart on an IFR flight plan into controlled airspace without receiving an IFR clearance from someone air traffic services. Some pilots never have to think twice about where they are going to get their clearance, but they probably always operate from the same few airports and they know the local procedures. The way you get a clearance in Canada can be quite varied, and when the airports you operate out of are that varied sometimes it's tricky.
If it's a busy airport, there may be a dedicated clearance delivery frequency published in the CFS and printed on the departure plate. You tune them up and call for clearance, and if you've filed your flight plan properly they usually have it ready and waiting for you, something like, "ATC clears Flashcube Three to the Peace River airport via the Moose Three departure, flight planned route, maintain 5000', expect higher five minutes after departure, contact departure airborne 125.725, squawk 3671." You copy that all down as they say it, say it all back to them, and when they say, "readback correct" you have a departure clearance.
If there is no clearance delivery frequency, you call ground instead. Both clearance delivery and ground are manned by people in the air traffic control tower at the airport, and often both frequencies will be covered by the same person at once. You discover this when you switch to ground for taxi clearance and get the same guy, or sometimes the clearance delivery guy gives you a taxi clearance with the departure clearance and tells you to monitor ground while taxiing. Ground may send you back to clearance delivery if the IFR data people discover an error in your flight plan, but eventually you switch to tower, who may give you an amendment to your departure or change the assigned departure frequency before clearing you for take off.
If the airport is uncontrolled, but there is still an FSS on the field, you usually call them for your clearance. You include in your request for clearance the runway you intend to depart from, because at an uncontrolled airport the pilot decides that, and that may influence the clearance you receive. They phone IFR data and IFR tells them your clearance, then they read it to you, you read it back to them, they tell you you have it right, and then you have the clearance. After that you just tell the flight service specialist when you're taking off, and off you go.
If there's no FSS on the field, but there's an RCO (Remote Communications Outlet)--a relay that lets you talk to a flight service specialist who is somewhere else--you may call them in exactly the same fashion. Sometimes pilots don't even know whether I'm talking to a remote or local FSS, which can be amusing when they ask "is it okay if i park here?" When I landed at one airport recently I was talking to an FSS specialist as I landed and I asked him when I reported clear of the runway, "do I call you for my clearance in the morning?" He said yes, but when I called his frequency in the morning the specialist working it told me to get my clearance from Centre, unless I couldn't contact them from the ground.
If there is no air traffic services agency reachable at all from the ground, I may be able to get my clearance by phone from the IFR flight planning folk, or possibly through the regional FSS people at 1-866-WX-BRIEF. You just keep calling aviation-related numbers until someone consents to give you a clearance. A clearance received by telephone (and some received by radio) will have a clearance valid time window, so you can use it between two zulu times, but if you miss your window and do not depart by then, you have to call back for a new clearance. So a request for a clearance should include an estimate of when you'll be ready to go, especially if it doesn't match your filed departure time exactly.
If you have no usable frequency and no telephone service, this happens mostly to pilots of amphib aircraft or others who used really remote and unserviced strips, you can't get a clearance before departure. You can then get a clearance in the air, but in that case you have to either depart VFR and remain in VMC until you have received your clearance, or depart on uncontrolled IFR and remain clear of controlled airspace until you have a clearance. Some people do this even when there are facilities on the ground where they could have received a clearance, but they find it to be a time savings. In the US if you do it without even having a flight plan filed it's called a "pop-up clearance" and is not abnormal there, but in Canada filing a flight plan in the air when there were facilities to do it on the ground is considered poor airmanship, and it's rude because it jams the frequency and puts the workload of accepting the flight plan on people who have more urgent work to do. You can see where this is going, with oblivious US pilots annoying Canadian flight service specialists and other pilots when they do here what is perfectly normal at home. As far as I can tell, the standard Canadian behaviour of flying cross-country VFR then calling approach to land VFR at a busy airport is the US equivalent. US controllers really don't seem to like that. Would they prefer me to pick up an IFR clearance before approaching their airspace?
The last option for IFR clearances in Canada is to conduct the entre flight outside of controlled airspace and not have a clearance at all. That's perfectly legal, and an aircraft can transition between IFR and VFR flight just by changing altitude by 500' and changing the transponder code. I haven't done that in a few years.