[Serusers] 486 Busy & 2 UAs with identical usernames

Friends, I am using 2 SJPhone UAs (B, C) on two different networks, that register with my SER server using the same (username=XXXXXXX, password). / INVITE / SJPhone - A (username: YYYYYYY) \ INVITE \ I dial the phone number XXXXXXX from a third SJPhone (A). When only one of the destination SJPhones is registered with SER, let's say B, then B rings. Then while B is ringing, if "ignore" button is pushed, a 486 is generated by B and sent back to SER. Then SER receives the 486 in failure_route and processes it. But when both B and C are online, both B and C ring. when "ignore" button is pressed on B, 486 is generated by B and received by SER but SER does not enter the failure_route. When ignore is pressed on the first destination, I expect SER to send a CANCEL to the second destination and receive the 486 generated by the first destination in the failure_route so that I can forward the first destination to voicemail! Can anyone share any insight as to why this is happening? Thank you for your help in advance ramin