From software 2.23, it is possible to DE-REGISTER the SIP account if the DECT handset is:
Today it can be changed only via auto-provisioning
0 = (Default) SIP account is not DE-REGISTERED
1 = REGISTER with Expires = 0 is send when the handset is out of range or switched off.
<param name="SipProvider.0.WithDeregOnHsNotReachable" value="1"/>