Need Help with odd Ademco Vista 20P problem

[safechecker@xxxxxxxxxxxx]

Hi.

I have an Ademco Vista 20P, using ADT monitoring.  I have had the
system a couple of years with no changes made to it.

For the past week, on about half of incoming phone calls, after one
ring the alarm board is answering the incoming call, giving the caller
about 2 seconds of a data squeal, then hangs up on them.  Again, this
happens only in about half of calls that are made to me, and it just
started about a week ago.  I know it is the alarm board doing this,
because I can hear the relay click when it answers the call and hangs
up.

My Vista 20P is very simple - nothing fancy...
-8 zones (all wired, no wireless) just doors and a few motion
-No X-10, no phone modules, no PC downloading setups, no wireless.
-Made no recent changes to phones or phone lines in house.

I rechecked lines on the RJ31X and everything appears OK, and wired
properly.  Line seizing capability works fine in testing alarm
system.  No line work was been done on the phones or the alarm.

On a whim, even though I do not use pc downloading with my alarm, I
viewed the settings for *95 and *94 and it is set to 15 rings, with no
download phone number (which I believe is correct.)

Except for having the entire control board replaced, I am stuck for a
solution as to why the alarm would answer half of my phone calls
(after 1 ring) and give a squeal then hang up.  Frustrating. Looking
for a solution or a way to bypass please.  Thanks!