What is the PSTN?

The PSTN is still there and still carrying calls. Most VoIP calls to an ordinary phone number cross into it at some point.

It sets the audio quality everyone lives with. Narrow bandwidth is why phone audio sounds the way it does, and why speech recognition has a harder job on a call than on a podcast.

Copper is being retired in many places, with providers moving customers onto internet-based lines. The number keeps working; what carries it changes.

A traditional phone call carries sound between about 300 and 3,400 hertz. Human hearing runs to around 20,000. The hiss that separates an S from an F lives mostly above 4,000 hertz, so on a classic phone line that difference is simply cut off before it reaches the other end.

You've worked around this your whole life without noticing. It's why people say "S as in Sam, F as in Frank" on the phone and never in person. Say a caller gives her last name as "Faber". Over the phone, "Faber" and "Saber" are close to identical, and a human listener guesses from context or asks. Speech recognition has the same missing information and less context to guess from. When you see a vendor demo recorded through a laptop microphone, remember that it was given sounds your real callers' phones will never deliver.

Before you cancel or convert an old analog line, walk around the building and check what's plugged into it. Phones are the obvious part. Other devices are easy to forget, because they've worked silently for fifteen years.

Alarm panels often dial out over a phone line to reach the monitoring company. Elevator emergency phones usually have a dedicated line, and local code may require one. Fax machines, older card terminals and some gate or door entry systems all expect analog dial tone too. Many of these can be moved to a cellular or internet adapter, but each one needs its own plan, and the alarm company wants to hear from you before the line goes dead, not after.

Make the list, call each vendor, and only then give your carrier a date. It's a dull afternoon of work that stops you from finding out during an emergency that the elevator phone has been dead since spring.

Forwarding works from a PSTN line the same as from anything else, using a star code at your carrier.

Recognition on narrow-band phone audio is harder than on clean recordings, which is one reason we publish no accuracy figure.

If your line carries one call at a time, that is the limit worth fixing first. Concurrency on the software side cannot help a caller who never got through.