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Question: no sms memory,how to expand it,what to do.

as soon as my inbox reaches 200 messages, the sms memory becomes full although i keep deleting messages from my sentbox and drafts.,and there is space for 500 messages,as the phone shows.
there is no setting through which i cn shift my message storage to memory card.that option is there in corby pro.is there a method to cure this problem?
please help me its urgent:(