IAX - 2 beeps every 15 seconds?

I have an IAX connection from my telephony PBX running asterisk to one
of my Allstar nodes running Allstarlink.

It works as expected when connecting using a analog telephone
instrument connected to an analog telephone adapter (ATA). However,
when using a Cisco VOIP phone, I hear two quick beeps every 15 seconds
while connected to the Allstar node.

Does anyone know what these tones might be and how to eliminate?


Dan K2IE