With your help i managed understand some basics and to do some setup with two android softphones.

The softphones register as 1000 and 1001 respectively. Calls between them are working. Now i try to implement the "door comm" scenario:

-> originate sofia/internal/1000%192.168.0.170 2001 XML default TEST TEST

will as expected call softphone 1000. When i answer, my dialplan extension 

<extension name="test">
<condition field="destination_number" expression="^(2001)$">
<action application="bridge" data="user/1001"/>
</condition>
</extension>

seems to be found and executed - but the softphone 1001 is not ringing now. After a while the call is discarded. Here's some Log output

freeswitch@MIT-T540-W8> originate sofia/internal/1000%192.168.0.170 2001 XML default TEST TEST
2014-12-15 23:12:21.265437 [NOTICE] switch_channel.c:1055 New Channel sofia/internal/1000 [9e94d0e6-80cb-42fd-af72-22140bfaa0e2]
2014-12-15 23:12:21.385007 [NOTICE] sofia.c:6716 Ring-Ready sofia/internal/1000!
2014-12-15 23:12:26.104794 [INFO] switch_core_media.c:5328 Activating RTCP PORT 4031
2014-12-15 23:12:26.104794 [NOTICE] sofia.c:7475 Channel [sofia/internal/1000] has been answered

+OK 9e94d0e6-80cb-42fd-af72-22140bfaa0e2

2014-12-15 23:12:26.104794 [NOTICE] switch_ivr.c:1854 Transfer sofia/internal/1000 to XML[2001@default]
2014-12-15 23:12:26.104794 [INFO] mod_dialplan_xml.c:635 Processing TEST <TEST>->2001 in context default
2014-12-15 23:12:26.104794 [INFO] switch_channel.c:3062 sofia/internal/1000 Flipping CID from "TEST" <TEST> to "Outbound Call" <1000%192.168.0.170>
2014-12-15 23:12:26.125865 [NOTICE] switch_channel.c:1055 New Channel sofia/internal/sip:1001@192.168.0.135:42990 [c49443c5-7bc0-4482-8851-8797b5e9102d]

freeswitch@MIT-T540-W8>