CLI output
--------------
    -- Starting simple switch on 'Zap/1-1'
[Jun  8 05:20:37] NOTICE[11366]: chan_zap.c:6351 ss_thread: Got event 18 (Ring Begin)...
[Jun  8 05:20:37] NOTICE[11366]: chan_zap.c:6351 ss_thread: Got event 2 (Ring/Answered)...
[Jun  8 05:20:42] NOTICE[11366]: chan_zap.c:6351 ss_thread: Got event 18 (Ring Begin)...
  == Starting Zap/1-1 at default,s,1 failed so falling back to exten 's'
  == Starting Zap/1-1 at default,s,1 still failed so falling back to context 'default'
[Jun  8 05:20:42] WARNING[11366]: pbx.c:2450 __ast_pbx_run: Channel 'Zap/1-1' sent into invalid extension 's' in context 'default', but no invalid handler
    -- Hungup 'Zap/1-1'


/etc/asterisk/zapata.conf
----------------
[channels]
language=en
usecallerid=yes
hidecallerid=no
immediate=no

signalling=fxs_ks
echocancel=yes
group=1
channel=1
context=from-zap-2

signalling=fxs_ks
echocancel=yes
group=2
channel=2
context=from-zap-1


/etc/asterisk/extensions.conf
-----------------
[general]
static=yes
writeprotect=no
clearglobalvars=no
[globals]
CONSOLE=Console/dsp
IAXINFO=guest
TRUNK=Zap/g2

; Ring SIP 101 when Zap1-1 rings
[from-zap-1]
exten => s,1,Wait,1
exten => s,2,Answer
exten => s,3,Dial(SIP/101,20)

; Ring SIP 102 when Zap2-1 rings
[from-zap-2]
exten => s,1,Wait,1
exten => s,2,Answer
exten => s,3,Dial(SIP/102,20)

[users]
; Extension 101 for dallas
exten => 101,1,Dial(SIP/101,20)
exten => 101,2,VoiceMail(101@default)
exten => 101,3,PlayBack(vm-goodbye)
exten => 101,4,HangUp()

; Extension 102 for nate
exten => 102,1,Dial(SIP/102,20)
exten => 102,2,VoiceMail(102@default)
exten => 102,3,PlayBack(vm-goodbye)
exten => 102,4,HangUp()