This sounds like a gotcha exam question. Hah.
Two-stage dialing means you call into a PBX and - hopefully after authentication, more on that in a bit - it gives you dial tone to originate a new call leg from it’s PSTN trunk. CUCM MVA is an example of this: you call the MVA access TN from, say, a hotel, login, and are then able to place an international call using the much cheaper rates of your SIP trunk vs. the hotel. This usually also has the benefit of showing your work caller ID to the called party.
It’s also possible for two-stage dialing to be accidental and a source of toll fraud. Cisco dial-peers will provide dial tone on the incoming call leg if the direct-inward-dial command is missing; this is almost always a very bad idea!
