[Asterisk-Users] Cannot dial two phones at the same time

[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.