POA Hangup

[toolbox212121]

I have a wierd issue on my switch. We have 3 phones (we will refer to them as A, B, C) all with the same number (DN 200) on a button on all phones. When a phone call is received on DN 200 phone A will answer, then phone B, then phone C. Its all fine and dandy until phone A or C hangs up. For some reason phone B will drop the call. Phone B can rejoin the call by pressing the DN button but the problem is when phone A and C hang up together then the call is lost for all parties. The pattern seems to be when the 1st phone to answer or the 3rd phone to answer hangs up it drops the 2nd phone.

Any ideas on why this is happening and what to do to fix it? Thanks

The switch is a CS-1000 and all the phone are 3904s with CLS POA enabled.