One trick is to forward a couple of buttons and label them as FWD All and FWD.  See the codes below for your system.

On the button labeled FWD, program the button so the phone forwards either as Busy Call Forward, No Answer Call Forward or Busy/No Answer Call Forward.

On the other button, program the button as Forward All Calls

When you press the FWD All button, calls to your desk will automatically forward to your mailbox.  When you press the FWD button, your phone will ring before going to voice mail

KX-TDA100  KX-TDA200  KX-TDA600

TDA.htm

KX-TD816  KX-TD1232

 KX-TD500

KX-TD.htm

KX-TD816  KX-TD1232

KX-TD500

Key KX-TD.htm

KX-T308  KX-T616

KX-T1232

 KX-T/KX-TA.htm

Forwarding Type

KX-TDA 50/100/200/600

KX-TD Digital Systems and KX-TA 1232

KX-TA 308

KX-TA  624 

KX-T Analog

 308/616 Systems

KX-T 1232 

Analog system

All Calls

Intercom *7102 XXX#

Intercom 7102 XXX

Intercom 711 XXX #

Intercom 71 XXX #

Intercom 711 XXX #

Busy

Intercom *7103 XXX#

Intercom 7103 XXX

N/A

N/A

See Chart Below

No Answer

Intercom *7104 XXX#

Intercom 7104 XXX

N/A

N/A

See Chart Below

Busy/No Answer

Intercom *7105 XXX#

Intercom 7105 XXX

Intercom 712 XXX #

N/A

Intercom 712 XXX #