The telephone circuit in my home has two phone jacks:

Jack #1 serves a Panasonic KX-TG5210 5.8 GHz Cordless Phone and a hard-
wire AT&T Trimline phone.

Jack #2 serves a hard-wired AT&T Trimline phone.

When an incoming call arrives:

only one phone rings
or only two phones ring.
or the loudness of the ringing varies from phone to phone.

How do I get all phones to ring and to ring loudly?