Have an ESI 50 system. 3 CO lines being used, using an after hours messaging service. When calls ring on line 1 they go out on line 3. During that time there's a loud feed back sound almost like an echo. Verified all 3 CO lines incoming and outgoing and all is clear. Verified the after hours service line and is clear also. Moved line 3 to line 4 and was clear for a while and started bleeding over to line 2.
Swapped the system for a new one and the feed back seems to have amplified. Checked voltage on my lines and line 1 and 2 have 21 vac and line 3 has 20. It should be 24 vac according to the diagrams.
Am lost and continue researching this issue, any thoughts?