International long distance service , often provided by the domestic long distance company, allows dialing phones in other countries. Some has to do with the service end of things, some with hardware. Trunks are the logical paths that calls can take between the business and the Central Office owned by the local or alternative telco. There may be a separate pair of wires for each trunk, or multiple trunks may be multiplexed onto a single set of pairs. The total number of trunks a business has determines how many people within the business can simultaneously be on the phone with callers outside the organization.
Standard trunks can be both inbound and outbound.
The telco sends the set of destination digits for incoming calls down the DID trunks to allow the businesses phone system to route calls. DID trunks allow inbound calls only. Lines are the actual wires that connect the local telco to the business. Extensions are the physical phones within the business. T1 is a special set of lines that allows 24 logical trunks to be multiplexed on the same line using analog signals.