In the USA the difference between a “line” and a “trunk” is basically this.
A line is what you receive dial tone over when you pick up the receiver of your home phone. A line also has a unique number assigned to it so people can call you.
A trunk is generally refferred to as an “inter-office” connection, that is telephone switch to telephone switch. Their is no unique number assigned or published, it can not be dialed or accessed in the normal sense and it is not dedicated generally to one subscriber.
It can be shared by the “lines” to make a connection from one switch to another.
Erlangs can be described as a unit or finite amount, for example 1 erlang may equal 1 hour of telephone traffic.
If you have 10 lines, you can´t put 10 erlangs of traffic on them because call setup time, busies, abandons, etc would only allow for example 9.6 erlangs mathmatically.
If you have 10 trunks you may put 9.8 erlangs of traffic because a trunk is controlled by the switching system computers, not a person with the phone in their hand thereby making the trunk more efficient.
The various references to erlang b, erlang c, etc. refers to the type of calling attempts you expect on the line/trunk(s). Erlang b allows for better utilization because it assumes a % of the callers will retry their calls if not successful, the normal erlang chart does not assume retries.
Hope this long winded explanation helped.. Ben B.