There are three ways you can depart from an uncontrolled airport with the intention to fly IFR.

First, if the weather is VMC, you can takeoff VFR and pick up your clearance using the departure frequency on the Jepp chart. This is the best way to expedite departure.

Second, if the weather does not permit a VFR depature, you can call FSS station. Don't call the number 1-800-WX-BRIEF. You will find the actual number listed under FAA Phones in the AFD towards the back. The number is listed under "clearance delivery". You might talk to the same person but when he picks up the phone, he/she knows you want your IFR clearance. Be sure that your preflight is done and you are strapped in ready to go. The IFR window given is usually 5-10 minutes away from the time you hang up the phone. Be careful of two things. (1) Make sure you depart and call departure within the time frame window given. If you depart after your clearance void time, you may put yourself and someone else in danger. You have become illegal. (2) Be careful to maintain proper clearance to terrain/obstacle. Use the obstacle departure procedure. Look at the back of Jepp airport diagram all the way at the bottom. If there is nothing, there is no obstacle departure procedure. You are responsible for your own separation.

Third, you can pick up your clearance over the radio on the ground. If you look at your sectional chart you will see that some airports are class E on the ground. One of the requirements since it is a "controlled airspace" is to be able to have two way radio communication with the ground facility. So you can pick up your clearance on the ground talking to departure.

I will give you one example. I flew from Narsarsuaq to Kulusuk in Greenland. Everything is class G. There are no radars. When I picked up my clearance all I was told is: "you are cleared to Kulusuk, airborne frequency is....". That was it. No altitude, no transponder, nothing. In class G, you are responsible for your own separation!!! You should see normally on your Jepp chart a common air to air frequency. It will be your responsibility to communicate your station passage altitude and time or time to start the approach so as to avoid conflict with other aircrafts. And yes, your IFR clearance is still valid the all time. You are just on your own. That's real flying. We are way too spoiled here