Cannot host Multiplayer on Android 2.3.5 (HTC EVO)

When attempting multiplayer play on my HTC EVO (Android 2.3.5), others cannot connect to my server/world. HOWEVER, when my son hosts multiplayer on his Kindle Fire (unknown Android version, sorry), I can connect to him and we play just fine! This problem was first seen with 0.4.0, but the update to 0.5.0 did not improve anything.
A coworker also has an HTC EVO, and neither of us can connect to the other's world for multiplayer play.