Vulture

zapata.conf
-------------
[channels]

group=1

context=POTS-Incoming
signalling=fxs_ks
callwaiting=no
callwaitingcallerid=no
threewaycalling=no
transfer=no
echocancel=yes
echocancelwhenbridged=yes
relaxdtmf=yes
rxgain=1.5
txgain=1.5
immediate=no
channel => 1

----------------
extensions.conf
----------------
[general]
 static=yes
 writeprotect=no

[POTS-Incoming]
 exten => s,1,Answer
 exten => s,2,Playback(pbx-invalid)
 exten => s,3,Hangup
 exten => h,1,Hangup