Scott - Is your other VoIP system a Cisco CallManager/Cisco router system?
CSCdv71088 - CallManager cannot do a hookflash on a fxo line with Call Waiting.
Release Note:
Symptom: Ip phone cannot answer call waiting beep sent by the telco
Problem: CallManager does not have the means to send the H245 message to the gateway to answer the Call Waiting call
Workaround: Create a hunt group of FXO so the call always comes in on a new trunk and now use Call Waiting on the PSTN (use CallManager call waiting).
http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCdv71088
Regards,
Michael.