TOYOTA 2010 Sienna - Manual (Page 368)

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3-4. Using the hands-free phone system (for cellular phone)
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Call history
Up to 5 phone numbers can be stored in each of the outgoing and incoming
call history memories.
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When talking on the phone
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Do not talk simultaneously with the other party.
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Keep the volume of the received voice down. Otherwise, voice echo will
increase.