Usually on hook telephone line voltage around 40V DC +/- depends on exchange, voltage will drops below 12VDC if its half hook..You will get engage tone/usage tone if voltage drops less than 12V, in this condition you wont get any incoming calls.
You need to open the printer formatter to locate the modem where telephone line inserted..
Let understand why this malfunction happens..
The ring voltage between the line is +120V AC, but it should not exceed more than 200V.. To protect the circuit, line will guarded by component called "VARISTOR". The Varistor will burns if voltage reaches specified or more voltage across it.. Once varistor burns U will get engage tone always... Basic multimeter is enough to check the varistor, put multimeter in diode mode, test both ends of varistor, it should not show zero value or beeping. Some times it may burn, blast or bulge. Remove the faulty component from the circuit with the help of soldering iron, connect the line & verify, if line is good replace with same rating of component.. Chk for the ring & dial tone...
Req tools: Multimeeter, soldering iron, little paste & Led, screw driver set for opening the printer..
Depends on design varistor may fix accordingly which we need to find out the location.. But its always fixed between line in path. pl see the attached sample diagram.
if you are non technical pl don,t try it...Best of luck..
Ex: Varistor
Damaged/burnt varistor