[App_rpt-users] 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? 73, Dan K2IE