You want to be Dr. Frankstein?
With a single ACS R6, 3x8, one outside line on CO-1
Ext. 17 is the courtesy phone. It has no lines assigned to it, and it is programmed to Hotline to Ext. 16 in #603
Ext. 16 is a line cord into CO-3
Ext. 15 is programmed with only Line 3, and DELAYED ring
Ext. 15 is a line cord into CO-2
Ext. 14 has Line 1 and Line 2 assigned to it, is the Line Coverage Extension for CO-2 in #208, has Remote Call Forward allowed in #322, has the cell phone programmed into its Feature 99 speed dial bin, and is forwarded to 99 (Feature 11-14-99)
Ext. 10 has Line 1 and Line 3 assigned to it. Auto Line Select does NOT include Line 3, and Line 3 had Unique Line Ring 8 assigned to it #209
User lifts the handset at Ext. 17. It hotlines into Ext. 16, which makes CO 3 ring. CO-3 is on Ext. 10 with a unique ring tone, if the guard is there, they simply pick up the line and talk to the caller. If the guard is not there, CO-3 starts ringing at Ext. 15 after 4 rings, which rings into CO-2, which is owned by Ext. 14, which is RCF'ed to the cell phone. Ext. 14 grabs Line 1 and dials the cell phone, the call stops ringing at Ext. 10's CO-3 button and is now ringing at the cell phone.
You HAVE to have reliable disconnect to tear everything down when the call ends.