Did, Sip Help

I need a little clarity on did’s and sip’s.

True or False: A “did” is just a phone number, and a sip is a way of connecting the did to Asterisk? So if I buy a sip, I still need to buy a did?

Also from what I’m reading, if the above is true, if I buy one sip trunk (I think also called a port or channel (y/n)) and two did’s. Those two did’s can’t call at the same time, nor could I have two people call on the same did at the same time. I would have to buy as many trunks (ports, channels) as I want concurrent calls, correct?

Thanks! Sorry for the newb question >.<

Yes, the DID (Direct Inward Dialing) is just a phone number, by itself it’s not attached to anything. This is normally provisioned by someone/a company with PSTN(Public Switched Telephone Network) access. SIP(Session Initiated Protocol) is just the protocol. With most VoIP providers you don’t so much “Buy a SIP trunk” or “Buy a DID” but rather “lease” it’s use .

To get incoming calls, from the PSTN via VoIP you’re going to need a DID. For outgoing VoIP calls you’re going to need a provider to terminate your SIP calls on the PSTN. Hope this helps.