Confusion between analog & digital signal and modem
I agree that you are confused.  A telephone line is a strictly analog connection.  The typical connection is shown in the picture below (from https://pbxbook.com/other/trunks.html#4 )

Unless you are using a digital telephone, voice is not converted to digital until it gets to the Central Office (CO).  A a modem modulates digital signals into analog signals that fit within the frequency bandwidth of the analog line.  Another modem at the other end demodulates the analog back into digital.