Hi
This concept of channels is something of a throwback to traditional telecoms, and I am not sure I like where this approach seems to be going.
In traditional telephony, someone would sell you say an E1, which was 30 channels - it's 30 channels because the technology of E1 does not allow more than 30 channels to be squeezed down that piece of copper.
In SIP, the concept of channels does not exist, how many channels can be sent to your PBX depends on your bandwidth, and your server's ability to process those calls.
So given that the telephony business is the only business I know where the prices only every go down, and rarely up, the carriers have to think of new ways of charging you, and charging on the basis of "Available Capacity" seems to be the way they are going.
If you have a large number of customers, with a substantial amount of traffic, you may consider moving into being your own Telecoms Provider. With larger amounts of traffic, you are working in the wholesale environment, where calls are inevitably x cents per minute, with to all intents and purposes, no simulated limits on capacity, although if you are likely to have a peak, they would like to know about it so they can plan for the future.
Finally, as an aside, it is interesting to note, as well as being great for the ego, that a number of the SIP providers which have been mentioned on these forums have either been our customers for whom we have installed a system and provided training and support, or we know they are using A2Billing, so if you were to take this step, you would not be alone.
Yours
Joe