Correct Flash Duration to Connect to Waiting Caller (Xfinity Voice Service)

 I have call waiting service on my Xfinity Voice phone account.  If I'm on a call and a second caller is calling, I hear a notification chirp that there's a call waiting. I have difficulty connecting to the waiting call.  There's a "flash/call waiting" button on my phone's handset.  But when I press it, the call usually stays with the first caller, despite repeated attempts to switch.  Sometimes, pressing the button ends up disconnecting the original caller and all I hear is a dial tone.  My phone system's default  flash signal duration is 700 ms, but I can change that to numerous other values between 80 ms and 900 ms.  No one at Xfinity can tell me what duration is most compatible with their call waiting software.  Does anyone reading this forum know the optimal setting to provide optimal reliability when trying to "flash" over to the waiting caller?