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I have finally found a solution for the problem. Here it is: Using Wireshark to sniff the UDP broadcast from the emulator I noticed that the destination address is incorrect - its "1.0.0.127". This address is not routable so the datagram sent from the one emulator will never reach the other emulator. I might have missed it but I cannot see any command line argument to set when launching the emulator to specify the destination address. I changed my development laptop to use a hard-coded IP address instead of DHCP. I then added 1.0.0.127 as another IP address to my TCP/IP settings. The address is now routable. Restarted both emulators with no changes to the port settings (/sms-source-port and...
Full answer by Will777: https://stackoverflow.stackexchange.com/a/11052449 (CC BY-SA 3.0).
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