Sip registration ring group


I have raspbx with two mobile sip phones (csipsimple app on android phones) and an analog phone plugged in an linksys spa3000. All incoming calls are directed to a ring group with every phone and a ring all strategy.

When nobody is home, meaning when no mobile sip phone are registeredn I would like the ring group to hangup calls even if the analog phone is registered .

Is that possible ? how should I proceed ?

Best regards,
Bidule from Guadeloupe