Let's start with a scenario where I have one BT analogue line connected to a single handset. I can make or receive exactly one call at any one time. If I make a call, any new caller will get a busy tone. If I receive a call any subsequent callers will also receive a busy tone.
Now expand the system to have a primary rate circuit with 10 channels enabled going into a PBX with one extension. Assuming the equipment can handle it, I can receive or make up to 10 calls at once, albeit with all but one on hold.
In essence the number of simultaneous calls I can handle is limited to the number of circuits; one in the first scenario and 10 in the second.
Now moving forward into the VOIP world, I have a single VOIP account connected to a single handset. Assuming all the necessary VOIP features are enabled and the handset can cope I could have received a call, had another put into call waiting, put the current call on hold, made an outgoing call and so on. Basically there is no VOIP limit to the number of calls coming in or out via the single SIP channel and single account.
So, in the BT way of doing things, we had a limit imposed on us by the number of circuits which we could use for outgoing calls which we had to share with the incoming calls. In the VOIP way of doing things we need one account, one IP connection to VOIP, any number of extensions, lots of PBX features and we can then make any number of calls in and out.
Is this correct?