Line Reversal was used with Analogue Networks. Classic Coin and Fee Checks with Payphones.
For Signalling C7 / ss7 is ubiquitious. Some Networks (legacy
requirements) still support R2 MFC signalling.
For Landline solutions with a Local Loop (Two-Wire interface), a line reversal is used to provide “wetting current” before CLI is sent to the CPE. Line Reversal, FSK (CLI) followed by the Ringing Current (from the CLIC).
VBR/ Wallis Dudhnath