I lost a girl by doing the same mistake you're doing. I too hate phone calls, even setting up an appointment with the doctor would take me an hour to prepare, I'd even have to write what I was going to say on a notepad and just read it out over the phone because my brain malfunctions. I too never call anyone really.
Which is funny, as I'm the most vocal person in the room, I usually do the talking when I'm in company of friends (guys and girls). Had a girl who wanted me to call her and I did, but not often enough. I had a fear that I'd run out of things to talk about and that there would be silence and that she'll find me boring over the phone (which is impossible face to face).
As much as you hate it, even if you explain it to her, it won't change things. She wants your attention, she wants to hear from you. It's your girlfriend, she wants you. Call her, show her you're thinking of her. Don't loose your girl like I did.