asterisk users - Apr 2020 - /outgoing/ .call files and RetryTime problem

asterisk-16.8.0

Hi

I've set up a callback script to retry a number if it's busy, but as
I watch the console output asterisk seems to rush 3 or 4 calls at
once before waiting the RetryTime of 20 seconds that I've set.

The script:

-----8<------
CALLERID=$1
EXTENSION=$2
TEMP=`mktemp /tmp/call-XXXXXX`.call

  cat <<EOF > $TEMP
Channel: IAX2/account at provider/44${EXTENSION}
Extension: $CALLERID
Callerid: <my home phone number>
Context: out
WaitTime: 15
MaxRetries: 9999
RetryTime: 20
Priority: 1
EOF

mv $TEMP /var/spool/asterisk/outgoing/
-----8<------

Some cut log output to show the timing:

-----8<------
[2020-04-23 13:05:38.104] NOTICE[23290]: pbx_spool.c:450
attempt_thread: Call failed to go through, reason (5) Remote end is
Busy
    -- Attempting call on xxxxxx at out:1 (Retry 327)
    -- Called xxxxxxx
    -- Call accepted by 1.2.3.4:4569 (format alaw)
    -- Format for call is (alaw)
    -- xxxxxxxxxxxxxxxxxx is making progress
    -- xxxxxxxxxxxxxxxxxx is busy
    -- Hungup 'xxxxxxxxxxxxxxxxxx'

[2020-04-23 13:05:38.468] NOTICE[23316]: pbx_spool.c:450
attempt_thread: Call failed to go through, reason (5) Remote end is
Busy

[2020-04-23 13:05:41.547] NOTICE[23614]: pbx_spool.c:450
attempt_thread: Call failed to go through, reason (5) Remote end is
Busy
-----8<------

So there it rushed 3 calls through in a few seconds without waiting,
and it will do that constantly, but after a `rush' it will wait the
20 seconds before retrying.

Any advice appreciated.