Так, одновременный вызов двух сразу телефонов без издевательств с диалстрингом делается так
<extension>
<condition field="destination_number" expression="999">
<action application="bridge" data="loopback/100500,loopback/100501" />
</condition>
</extension>
<extension>
<condition field="destination_number" expression="100500">
<action application="log" data="CRIT 100500 "/>
<action application="bridge" data="user/1002@${domain_name}" />
</condition>
</extension>
<extension>
<condition field="destination_number" expression="100501">
<action application="log" data="CRIT 100501"/>
<action application="bridge" data="user/1003@${domain_name}" />
</condition>
</extension>