Virtual fax is the right answer for most small businesses. It covers the real-world fax use case, which is sending and receiving documents by email.
A few things are different. If you still have a physical fax machine you want to keep, an elevator phone line, or an alarm system line, VoIP is not the right place for those. Analog is. Here is how we think about it.
Keep the fax machine
A physical fax machine needs an analog phone line. You can run it on VoIP with an ATA adapter, but fax protocol over VoIP is unreliable. Any packet loss or jitter and the handshake fails.
If you want a real fax machine on your desk, use a bundled analog line from your cable or internet provider. Most providers will sell you a voice line alongside your internet service for a low monthly rate.
If you do not need the physical machine, switch to virtual fax. It is more reliable than any fax machine you have ever owned.
Elevator phone lines
An elevator phone is a life-safety line. When someone is stuck and hits the button, the line has to complete the call to a monitoring service without retries, without failover, without a VPN reconnect.
Put elevator lines on analog. Either a POTS line from the phone company or a bundled analog line from your cable provider. Do not put them on VoIP, and do not put them on cellular backup for the main line.
Most building codes and elevator service contracts require a landline for this reason. Check your local requirements before changing anything.
Alarm system lines
Fire alarms, burglar alarms, and medical alert systems often dial a monitoring center over an analog line. Same reasoning as elevators. The signal has to make it through on the first try, and the alarm panel expects a POTS-style connection.
Leave alarm lines on analog. If your alarm company supports IP or cellular reporting, let them handle that upgrade on their own equipment. Do not route the alarm line through your VoIP system.
Why bundled is cheaper
Cable and internet providers sell analog voice lines as add-ons for a small monthly fee, usually much less than running a VoIP account plus a physical ATA device with custom firmware.
For a single elevator or alarm line, the bundled path from your existing provider is almost always the cheapest and most reliable option. It also keeps the life-safety and legacy lines out of your business phone system where they do not belong.