Typically, I would telephone ATC from the office to book out the aircraft (i.e. warn them when I'm about to go, tell 'em where I'm going, how many on board, how much fuel etc).
When I get to the aircraft, I then talk to ATC via a VHF radio; firstly dialling up ATIS for the weather and then Tower for start, taxi and departure clearances. I will later be told to change frequency to Approach/Radar once outside the aerodrome zone.
So, telephone call first, then radio. Thankfully, I have never been asked to make a telephone call after a flight; that's usually bad!
I carry a mobile phone with me (switched off!) just in case there's an emergency. If I had to make a precautionary landing for example, ATC may not be able to pick up the radio signal as VHF works on line of sight. I wouldn't want them to worry about me unnecessarily.
Cheers
Whirls