Okay there are many free softphones available. Callcenters do not want to buy one. Thing is when you buy dialer trafic from a person 🙂 [not a company], only offered codecs would be G729 and G723. Now free softphones usually do not have support for these proprietary codecs. Asterisk has to bear whole load of transcoding which significantly decreases server capacity. To escape this you can either use uncompressed G711 without transcoding and pay for bandwidth or buy proprietory softphones.
In reality not a single client of mine ever bought one and always used cracked eyebeam. I forgot to mention that you would have to install G729 either buy one from digium or use intel implementation and pay some royalty fee.
In your sip.conf enable only g729 and disable all other in global and peer sections. Do the same in your softphone settings.