What to check

The power supply, once started, only provides 20v for typically 1 minute before timing out.

900-VS4 video distributors have 1 amplifier for all 4 buffered outputs, it is likely that all outputs have failed, check the 20v + - input voltage and the incoming Coax.

Corrective Action

Test the coax going to the phone, disconnect coax and measure for resistance, should be about 75 Ohms.

Links

https://www.entryphone.co.uk/shop/productdetail/900-VS4