I remember that on the two keitais I've had (904SH and SH-01B), there was a setting to filter spam mail/SMS. When someone sent me a message from a web service - your response messages seem to be exactly this - the 904SH blocked them when ON. On the 904SH, the setting was available offline, but if I'm not mistaken, on the SH-01B, the setting required a Docomo SIM and working mobile data connection to turn ON/OFF. Just go through all your security settings - hope it's not tried to a Docomo SIM.
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