[Asterisk-Users] cannot dial two phones using zap

[Eddie]

I cannot dial two phones using zap at the same time.
One will ring but the other one hangs up.

zapata.conf

[channels]
context=default
signalling=fxs_ks
immediate=no
busydetect=yes
callprogress=no
echocancel=yes
echocancelwhenbridged=yes
usecallerid=yes
usecallingpres=yes
threewaycalling=yes
transfer=yes
callerid="Incoming" <20941261>
group=1
channel => 3,4

extensions.conf

[internal]
exten => 300,1,Dial(Zap/3/206&Zap/4/221,15)
exten => 300,2,Hangup

CLI

linux*CLI> dial 300 at internal
-- Executing Dial("OSS/dsp", "Zap/3/206&Zap/4/221|15") in new stack
-- Called 3/206
-- Called 4/221
-- Zap/3-1 answered OSS/dsp
-- Hungup 'Zap/4-1'
<< Console call has been answered >>

Please advice. Thanks.