Step1
First, find a portable hobby, by which I mean find a hobby that you can bring with you when you go places. If you're a woodcarver, sorry, you're out of luck. But for things like embroidery, knitting, crochet, puzzles, even video games, you can take good advantage of any downtime at your call centre job. Finding your niche is the first real step to using all those moments between calls to your full advantage.
Step2
Crafts are easiest to bring along, and by the end of a typical 8-hour shift, depending on how proficient you are at your craft, you can get a lot done. In between calls, work a little at your chosen portable hobby. You may not be able to get much done at a time, but by the end of the day, you'll find you've accomplished a fair bit. Much more, obviously, than if you hadn't brought anything to do at all.
I've knit entire scarves over the course of only a few shifts, and many people I work with have worked on crocheted blankets.
Step3
Some call centres are permissive about bringing portable video game systems to work, so long as the volume is kept off and you're not playing the game when you're actually on a call. Use this time to advance in whatever game you might be playing. Simple games are usually the best kind to bring, since they don't require a great amount of attention and yet are still enjoyable.
Step4
Bringing a book to read may also benefit you, especially at slow times, provided you're good at picking up where you left off, or else you might find yourself reading the same sentence over and over.
If you enjoy writing, write stories in an email to send to your home address, so you can have a copy of it there too, for later work.
If you enjoy doing puzzles, bring a puzzle book from home and work on them between calls, too. Drawing can also be accomplished quite easily, provided you're not bringing too many supplies along with you.