Answer on first channel only

Hi guys, I'm configuring a Trixbox machine with a TDM400P card with both channel in use for outbound calls.
Now the system is answering on both channels but I'd like to answer only on first one.
I have one Trunk configured in dahdi compatibilty mode (trunk/g0)
My chan_dahdi.conf is:
; Span 1: WCTDM/4 "Wildcard TDM400P REV I Board 5" (MASTER)
;;; line="1 WCTDM/4/0 FXSKS (In use) (SWEC: MG2)"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-pstn
channel => 1
callerid=
group=
context=default

;;; line="2 WCTDM/4/1 FXSKS (In use) (SWEC: MG2)"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-pstn
channel => 2
callerid=
group=
context=default

Any ideas on how to do it?
Thank you