Hi Sam, I think what your going through is a normal grief experience. When something tragic and unexpected happens I think it normal to feel the way you do. Your upset and trying to cope with it, but whether you picked up the phone or not, he probably would have still taken the pill.
It is very sad, and just be easy on yourself and give it some time. I hope you feel better about things soon. Also, feel free to talk about here as much as you need to.
A friend of mine died in a house fire, I felt the same way as you do, thinking if I was there maybe I could have gotten him out of the house. It was tragic and I was just coping with it, and Ive heard others talk about the same thing happening to them when they experienced something really tragic. Take care.