We get this question a lot, especially from fax machine vendors, sometimes from credit-card terminal vendors as well. They will generally tell the customer that their equipment doesn’t work well with digital phone lines, and they need to get an analog phone line.
All phone lines are both analog and digital. The difference is: where does the conversion take place? For a standard analog phone line (POTS – Plain Old Telephone Service), the conversion takes place at the Central Office (“CO”) operated by your local old-fashioned phone company. The line from your fax machine to the CO is analog – it’s a pair of copper wires. At the CO, the signal is converted to digital and transmitted via the PSTN (public switched telephone network), which uses the same underlying technology as the Internet to transmit the data to the CO nearest the destination phone or fax machine. At that remote CO, it’s converted again to analog and sent along the final stretch (called the “Last Mile”) to the phone or fax machine or whatever is connected to that phone number.
With a VoIP system, the conversion also takes place at the other end of a pair of copper wires; the difference is, that other end is in your office. It’s an analog adapter that converts voice signals to data signals and back again, very similarly to the way the phone switch at the CO does.
So why do so many vendors think you have to have a POTS line for a fax, postage meter, or alarm? Because many VoIP vendors do not do a very good job of configuring or conditioning their equipment to work properly with these analog devices. In fairness, that’s not a simple task. All of these devices require a real-time connection without gaps or static or echos. Getting that to happen reliably over the open Internet, which nearly all VoIP vendors use for their telephone service, is extremely difficult.
The best solutions use a combination of excellent end-user equipment, smart software, dedicated circuits (or at least VPNs – Virtual Private Networks) between the end user and the hosted services provider, and cleverly adjusted combinations of parameters at both ends to get the effect of a continuous real-time communication even when the underlying connection is not that way. The VoIP designer for one of the largest service providers told me “Fax is a dark art; it’s the occult science of VoIP.”.
Of course the bottom line is: it’s gotta work. So make sure your provider is either able to make your analog device communicate flawlessly over their VoIP system, or be willing to install a POTS line at no additional cost if that’s what it takes.