This would indicate an AC feedback into the phone. Since modems have a "bridge lifter" built into them, whatever is connected to phone side is taken out of the circuit. You can check this by verifying that there is no dial tone present on the phone. AC feedback must have a path to ground to cause the phone to ring continuously. The only path it has is through the speaker phone power suppy, unless the modems bridge lifter is defective. Most modern phones are equiped with polarity guards. Sometimes, but not always, a simple line reversal will correct the problem. You should try replacing the phone and/or the power supply if reversing the line does not solve the problem.
