The connector for the telephone line on a modem (the "Line Interface") is a very delicate piece of hardware and is highly susceptible to being damaged when plugged into non-standard telephone lines.

• A standard analog PSTN telephone line (found in most residential dwellings) has an electrical voltage of 48 volts DC, with a current between 20 and 100 mA.

• A digital PABX line (found in many hotels and office buildings) should have an electrical voltage of 48 volts DC (to emulate PSTN), but has the capability to spiral the current, from 20 up to 1000 mA.

All modems are made to work on standard analog PSTN lines, and the increased current of a digital PABX line will overload the line interface and damage the components. The normal symptoms of this are similar to lightning damage, in that the modem will no longer lift the telephone line to dial (resulting in "NO DIAL TONE" messages) or in some cases, the modem may no longer accept AT commands at all (resulting in "MODEM IS NOT RESPONDING" messages).

It is recommended that all users of modems check with the hotel or company whose telephone line they will be using, and make sure it is safe for modem communications. Most Hotels these days cater for data communications, and can supply either ISDN or analog PSTN lines on request. If you are not sure whether a telephone line is analog compatible, it is recommended to not use your modem, as you may cause permanent damage to the modem.

There are several devices you can purchase to help you with PABX lines. It is possible to obtain a line-testing device from telephone and computer communications stores. These are usually small devices shaped like a highlighter pen that have a green and red LED. When the green LED is lit, the phone line is okay to use. If the red LED is lit, the line is most likely PABX, and connecting a modem is not recommended. These devices cost around $30. For slightly more money, some companies offer couplers which go between your modem and a phone line to suppress current spikes. However, the modem may still not be able to obtain a dial tone on all PABX systems.

In addition some recent modem models have the ability to detect PABX lines and warn you to disconnect. Some still newer modems can suppress the current spikes and will allow you to connect through a PABX system. As with the coupler described above, you may not be able to obtain a dial tone on all PABX systems. However, these are new technologies and not yet widely available.