Cannot dial two phones at the same time

Hello everyone,
I’m new here.

My office has an existing / working PABX - Panasonic KX-TD1232
I want to intergrate with Asterisk
I have Digium TDM04 - 4 fxo

2 ext from Panasonic into Asterisk - ext 212 into zap3 & ext 211 into zap4

I want to call two phones ext at the same time - (206 & 221) so a call can be
connected at which ever ext picks up the phone first.

Here is my extensions.conf

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

Result:
only zap/3/206 rings
zap/4/221 won’t ring

linux*CLI> dial 300@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 >>

Since the first config not working, I tried :
exten => 300,1,Dial(Zap/3/206,10)
exten => 300,2,Dial(Zap/4/221,10)
exten => 300,3,Hangup

Result:
Zap/3/206 - won’t timeout / won’t stop ringing

Here’s my zapata.conf

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

Please advice. Thanks.

If it’s an option, I recommend on installing AMP.
Not only is it usefull to manage asterisk, the extensions.conf also give a great view on how you could create your own dialplan…

amp.coalescentsystems.ca/