Question on Find-me/SimRing

Newbie here...
When I chose sequential ring, where the second in the list is my cell phone, I noted this problem which may be a dumb-newbie issue:

I place a call from my POTS phone, not my cell phone, the VoIPo phone rings. I answer/go off-hook with the VoIPo phone. The POTS phone continues with the ring-back tone as if no connection is made.

Edit: I checked the settings for find-me sequential ring: 20 seconds for VoIPo, then 30 seconds for the 2nd number which is my cell phone. I wonder when that "20" begins relative to call setup?