Identify what uses the circuit
A line labeled phone may also support a fax, door entry system, payment terminal, alarm panel or modem. Record the attached equipment, service owner, called destinations, monitoring provider and power behavior. Check for parallel wiring and devices that seize the line ahead of a handset. Do not cancel the old service based only on a successful voice test. The acceptance criterion is the complete application, including its failure reporting and recovery behavior.
Choose an appropriate replacement
An ATA can be suitable for an ordinary analog handset when interface and service requirements match. Fax needs its own transport validation. Alarm and modem applications may need a vendor-supported IP or cellular communicator instead of an audio adaptation. Ask the responsible equipment vendor or monitoring operator to specify and validate the replacement path. A generic G.711 or T.38 capability claim is not evidence that a safety-related application is approved or reliable on that service.
Test the failure conditions
Plan controlled tests with the service owner for loss of power, internet interruption, device reboot and failed calls. Confirm how a fault is reported and who responds. Include backup-power duration for the ATA, switch, router and access equipment, not just the PBX. Keep circuit ownership and rollback instructions available through cutover. Retire the old path only after the agreed application checks are complete. Update labels and inventory so the next administrator can tell which services still depend on analog interfaces and which have migrated to a different transport.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.