If this object is enabled(1), and the call is recognized by the dialed number then calls are never accepted, and a callback is forced for incoming calls at once. The callback can be delayed for biboPPPRetryTime seconds by setting this entry to delayed(5). If the call is recognized by inband authentication (PAP or CHAP) then the actual connection is closed and a callback is performed at once. Setting this value to ppp-offered(4) allows a called peer to call back the calling site if offered by PPP negotiation. For PABX dialout a dialprefix is added to the number, if offered by the calling site (see isdnStkTable). If this object is set to expected(3), only one initial outgoing call is made expecting a callback. If this object is set to ppp-callback-optional(6), the CBCP option 'no callback' is also offered to the Windows client so that the user can decide wether he wants to be called back or not. Setting this value to vpn(7) a callback will be performed either via PPTP (if biboPPPLayer1Protocol is set to pptp-pns(20) or pptp-pac(23)) or via ipsec. Setting this value to expected_cbcp(8), callback is requested by using the Callback Control Protocol (CBCP) - same procedure as done by Windows Clients.