It doesn't matter what you use, just keep it separate from data traffic. If you are running traffic on 10.21.1.0, run voice on 2.0. If your entire 10.21 is carved up for data (which would be quite a large lan), then yes, use 192.168 or 10.22. The IP number itself doesn't matter, just keep it separate so it doesn't content (at least easily) w/ data traffic, and it allows you to more easily utilize QoS since it will be vlaned and subnetted different than the rest of your networks.
As the other poster stated, it really has nothing to do with the IP addressing. Its all about your subnetting, and even more importantly your vlans.
Here are some of the common practices:
One subnet for voice and one for data for each access switch.
One vlan for voice and one for data ideally applied to each access switch as well.
Voice and data traffic should be seperate, data side should not be able to affect voice.
Now, as for #3 I have found this is the best practice and it is what I think you will find others recommend as well. But it creates a problem when you allow users access to say the ccmuser site and other things. I think people will suggest you restrict access between the vlans / subnets to only allow the traffic you need. So maybe an ACL saying data subnet can access port 80 on the callmanager or something.
As for the ip addressing Trendkill is right. It doesn't matter. Ideally I would use something in the 10.22.x.x range if there is an unused subnet. But even the 192.168 would work.