TOYOTA 2011 Avalon - Manual (Page 336)

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3-4. Using the hands-free system (for cellular phone)
AVALON_U (OM41428U)
n Call history
Up to 5 numbers can be stored in each of the outgoing and incoming history
memories.
n When talking on the phone
l Do not talk simultaneously with the other party.
l Keep the volume of the receiving voice down. Otherwise, voice echo will
increase.