If you have a wiring cabinet like some modern houses do, the incoming line "feeds" the distribution wires that go to all the jacks. Disconnect the incoming line at the Network interface outside, and disconnect it in your wiring cabinet for good measure.
Older houses, the wire comes in from the outside, and then goes to the first jack then to the next etc.
or some combination of serial (jack to jack) and star (spoke and hub) type connections.
If you think how your landline type phones get a signal in your home. think of the wires as water pipes.
If there is a connection, the water can/will flow.
When an AT&T type land line is disconnected it is dead, no dial tone, ie no calling out, no 911
It's not like a cell phone where you can still call 911.
However
If you disconnect your previous land land (AT&T etc), the field technician does not always disconnect the drop from the pole or pedestal to your house. Then someone down the street might move in. If the telco uses that same pair of wires to give the new person service, your line would then have dial tone ie 48V DC talk battery or @90V AC ring voltage, not to mention possible induction on the line.
If you had phone service thru a CATV provider, the signal either came from a jack on the back of the cable modem, or outside the house from a special box that provides dial tone and ringing voltage.