Modems have two phone jacks on their backs. One of these jacks connects the modem to the wall. The other jack is designed to allow you to plug a telephone into the modem for voice calls. If the phone cord that plugs into the phone jack in the wall is plugged into the wrong port on the modem, you won't be able to make or receive calls with the modem. Another reason you might not be able to dial out is that the modem may be plugged into the wrong type of phone line.
Many businesses use digital phone lines.
You can't connect an analog modem to a digital phone line and expect it to work. Plugging a modem into a digital phone line can damage the modem because of the differences in voltage on digital and analog phone lines. One way to determine if your business is using a digital phone line is to plug in a regular phone and see if you can get a dial tone. Instead, use a phone from home. If you don't have a spare phone that you can use to test the line, check to see if the phone in your office has an LCD screen on it.