You need to use the C-1000B in the “Analog Station Mode”.  This is described in the product manual on page 4, section H.  When the C-1000B is used in this mode, your ringdown circuit would actually connect to terminals 4 and 5 (line out to phones) of the C-1000B but there is some programming to put the C-1000B in this mode that needs to be done first.  To access programming, connect a phone or butt set to terminals 4 and 5, set the talk battery switch to on, turn on dip switch 2 and go off hook.  You should hear 2 beeps (if you happen to hear two beeps that keep repeating over and over, the feature has already been programmed).  After the 2 beeps, enter “*5” and you will hear 2 more beeps.  Hang up the phone or butt set, turn dip switch 2 off and the talk battery switch off.  Now connect your ringdown line to terminals 4 and 5 and the call box to terminals 8 and 9 (door box 1).  Press the call button to test the operation and it should work properly.  The person that answers the call would dial “**” to provide a contact closure for the door or gate.