Telephone Line Backup Switch For Alarm System

This unit is intended for use with an alarm system that sends alerts over a phone line. Normally it will connect the alarm to a primary phone line (such as a landline). If that line drops, the alarm is switched over to another backup line (such as a fixed cellular unit, not included).  It comes with the necessary 12VDC 1A power adapter. It has an internal rechargeable battery to keep the unit active in case of a power failure.
This unit is believed to work, but I no longer have phone lines to test it so NO GUARANTEE.
For further details see https://shieldtechsecurity.com/phone-switch-primary-phone-line-to-backup-cellular-communicator.html