features.conf
[general]
parkext => 700                  ; What extension to dial to park
parkpos => 701-720              ; What extensions to park calls on. These needs to be
                                ; numeric, as Asterisk starts from the start position
                                ; and increments with one for the next parked call.
context => parkedcalls          ; Which context parked calls are in
[featuremap]
automon => *1                   ; One Touch Record a.k.a. Touch Monitor

extensions.conf

[from-internal]
exten => 0,1,Dial(Zap/g1/ww0,,wW)
exten => 911,1,Dial(Zap/g1/ww911,,wW)
exten => 500,1,Meetme(500)
include => parkedcalls
exten => 800,1,Directory(default|from-internal|f)
exten => 900,1,VoicemailMain(${EXTEN}@default)
# INCLUDE /etc/asterisk/cellphones.conf
exten => _xxx,1,Dial(SIP/${EXTEN},20,wW)
exten => _xxx,n,Voicemail(${EXTEN}@default)

CLI
  -- Executing [700@from-internal:1] Dial("SIP/129-081f13d0", "SIP/700|20|wW") in new stack
[Apr 21 14:22:30] WARNING[5815]: chan_sip.c:2907 create_addr: No such host: 700
[Apr 21 14:22:30] WARNING[5815]: app_dial.c:1196 dial_exec_full: Unable to create channel of type 'SIP' (cause 3 - No route to destination)
  == Everyone is busy/congested at this time (1:0/0/1)
    -- Executing [700@from-internal:2] VoiceMail("SIP/129-081f13d0", "700@default") in new stack
[Apr 21 14:22:30] WARNING[5815]: app_voicemail.c:2850 leave_voicemail: No entry in voicemail config file for '700'
  == Auto fallthrough, channel 'SIP/129-081f13d0' status is 'CHANUNAVAIL'
asterisk1*CLI>