The COMx port that the modem is set to may be used by another card in the computer.
Change the COMx port of MT5634ZLX/E to a different unused port. It might be best to set it to COM3 or COM4. Even if you are not using a serial port which is set to the same COMx port as the modem, it can still interfere with the modem. Be sure to update your software COMx port setting as well.
The Interrupt that the modem is using may be the same Interrupt that another card is using.
Change the IRQ settings to a different, unused, IRQ. Even if you are not using a serial port which is set to the same IRQ as the modem, it can still interfere with the modem.

MultiMobile User Guide

Introduction

This chapter provides general and specific problem solving steps. Below are a number of examples of Problems (P:), Causes (C:), and Solutions (S:). If your MT5634ZLX/E is not working properly, look up the problem and solution here; if unsuccessful, call Tech Support. At the end of this chapter, there are some specific steps for resolving IRQ and COM port conflicts.

P:No Response to AT Commands

C:

S:

C:

S:

P:The Modem Does Not Execute the Command Line

C: Make sure you are typing ‘AT’ at the beginning of command line (and hit Enter at the end).

C: Make sure the modem is not in Data Mode. Type +++ if necessary.

C: Make sure your software is set to the same COMx port as your MT5634ZLX/E.

P:The Modem Gives an ‘ERROR’ Response After an AT Command was Executed

C:Make sure you did not type an invalid command.

C: Make sure your command line is 40 characters or less.

P: The Modem Goes Off-Hook and Disables the Telephone Line.

C: The modem may be set to auto-answer the phone when it rings.

S: Disable auto-answer by typing ATS0=0 then Enter at the command line.

P: The Modem Does Not Auto-Answer the Phone

C: If necessary, make sure the software is configured to auto-answer the phone.

A: To set auto-answer mode on from AT command mode, type ATS0=n then click Enter. The n stands for which number of rings the modem answers. For example: ATS0=3 causes the modem to answer on the third ring.

P: The Software Does Not Control the Modem Properly or Can Not Detect the Modem

S: Make sure the software was set up correctly. Check the initialization string and dial string.

C: Some TSRs (programs that stay in memory after they are loaded) may conflict with the communications software.

S: Try starting your computer without loading any TSRs.

P: The Characters on the Screen Are Doubled

C: Both the modem and the software have the echo feature turned on.

S: Since only one needs an echo, turn the software echo feature off.

C: The remote modem is echoing your typed characters.

S: Type ATE1 then Enter at the command line. Then turn off the software echo feature.

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Multi-Tech Systems MT5634ZLXE Modem Goes Off-Hook and Disables the Telephone Line, Modem Does Not Auto-Answer the Phone