There is at least one other factor that should be checked first.
Telephone companies define a "ringer equivalency number" for each device that rings when your telephone number is called. This number represents the current each device draws. The maximum number of RENs per circuit is going to be a number less than 4.0 or 5.0. The exact number is set by the capabilities of your local phone carrier.
As more devices are connected to the phone line the total REN value increases. When the total REN value starts to approach the maximum value set by the phone company, strange things can start to happen. One of them is that when you receive an inbound call, the modem may not detect there is a call at all. It therefore does not go "off-hook," and you do not receive the call.
The modems included with Apple computers usually have an REN of between 0.7 to 0.9. Third-party modems have similar values. Some typical values are:
- iMac internal modem: 0.3
- Global Village Bronze: 0.7 (used on most Performas prior to 1995)
- Global Village Gold II for Performa: 0.8 (used on the Performa 6100 and some 630 series machines)
- Global Village Gold IIv (used in the Performa 6200 and 5300 series): 0.80
- Global Village Platinum V (used in the Performa 6300, 6320, 6290, and some 6400 series): 0.88
- GeoPort Telecom Adapter: 0.7 (stand-alone kit; also included with some Performa 6116s)
- GeoPort Telecom Adapter II: 0.5 (stand-alone kit)
The exact number can vary throughout the manufacturing life of a product, by usually not by more than 0.1. The number for your particular device is usually going to appear on the modem's FCC ID plate. In the case of internal modems, it will be on a placard on the modem card itself (usually not visible from the outside).
Telephones can have values of between 0.7 to 3.5. The exact REN is usually going to be stamped on the bottom of the telephone base, right in the plastic, or on an FCC ID plate, if one is present.
To test whether your modem is not answering because of too many telephones, fax machines, or modems attached to your phone number, proceed as follows:
1. Disconnect ALL additional phones. It is not sufficient to simply turn the ringer off. Remove them from their wall sockets.
2. Plug in one phone into the telephone wire servicing your computer. Verify that you get a dialtone.
3. Configure your fax software correctly. The details will vary depending on what computer you have.
4. Attempt to receive an inbound call.
If you receive a call successfully, go back and determine the RENs of each device (including the modem) you have attached to your phone number. If the value is above or near 4, call the phone company and see what their maximum total REN value for your central office is.
The phone company may have additional suggestions or recommendations on how you can overcome this problem, but odds are, you may need to invest in another phone line.
For more information on the Ringer Equivalency Number for the iMac internal modem, please see the following Knowledge Base article:
24643: "
iMac: Ringer Equivalency Number (REN) For Internal Modem"