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.
On the All messages screen, tap a contact (or phone number) to display the exchange of messages with that contact.
Tap the box that says Tap to add text, then enter your reply message
and then tap .
Select another phone number of this contact, and then tap OK.