Yes, this is a plain knitting post.
P was the one who first asked me for one, and I figured out a pattern, and now I need to make myself a new phone cozy, because I've been giving them away! BaronessMartha has one, and I just sent the replacement I made to someone else.
I truly don't know how to describe the cast-on, which is the only tricky part about this pattern. It's a figure-eight three needle cast on, so there are no seams. I've looked, and not found a good description on-line; essentially you put a slip-knot on one needle, then do a figure-eight around and between a pair of double-pointed needles and treat those loops as your cast-on.
At my gauge, 15 stitches to a side is perfect - but I know I knit loosely. I'm using sportweight yarn and either #3 or #4 double-pointed needles, working in the round. As soon as the bag created is the length of the phone, I divide by the number of holes I want, and do a yarn over followed by K2Tog every few stitches. Then knit two or three more rounds, and cast off. I tend to crochet the drawstring with a simple chain stitch, because it's quick and easy for me; it could be braided or an I-cord, I'm sure.
If you're as clumsy as I am, this might save your phone when you drop it. *smile*