ive had a lot of experience with call centers and megacall is by far the most user friendly and easiest to use. It has multiple modules which allows you to do anything from simple phone numbers to tracking clients info.
And its really easy to setup and get going. With megacall you dont even need to use a real phone number and you can do it on your cell phone if you so wish! There is a live demo available which is great for anyone who wants to see how it works before purchasing. Megacalls support is amazing and there is a wealth of information to learn about each module. I highly recommend this software.