Every building has them: the fax line in the back office, the fire alarm panel dialer, the elevator phone. They ride on copper, they are invisible until they fail, and they are exactly what gets forgotten in a phone switchover.
Fax needs a specific protocol setting to survive on modern lines. A generic VoIP adapter garbles pages and you find out when a contract does not arrive.
Alarm and elevator lines have life-safety code behind them. The replacements are purpose-built cellular units, and your alarm company has to verify the connection after the swap. Skipping that step can fail a fire inspection.
When we quote a copper replacement we walk every line in the building first, because the $20 forgotten line is the one that causes the $2,000 problem.