MASTER YOUR DEVICE - DevDB

Messages 1 5
Replying to another phone number of the contact
When a contact has multiple phone numbers stored on your phone,
you’ll see the particular phone number used within the message.
There’s also a bold letter before the phone number to indicate the
number type (for example, M stands for Mobile).
Keep in mind that your response will be sent to the phone number of
the latest message you’ve received from this contact.
If you want to change the phone number before you send out your
reply, press MENU, then tap More > Recipient phone number. Select
another phone number of this contact, and then tap OK.