Why is SMS to contact group not using contact's default number? Sends to wrong number Scenario: contact group ABC contains 3 contacts, A, B and C. I regularly text the group, and it works fine. One day, contact A gets a new phone and a new SIM. As is my habit, I retain A's old number for a while, just in case, and add the new number to A's contact record and mark it as the default number. 

From this point, if I phone contact A, it uses the new number. Fine.

But now, whenever I text contact group ABC, it still sends to A's old number. Not fine.

I have searched for any information or fixes but drew a blank.

This is a Samsung Galaxy Ace 4 btw.
The reason is that the existing conversation thread for contact group ABC has hard-coded knowledge of the phone numbers used, rather than a reference to the contacts. Therefore after setting a new default number for contact A, texting group ABC using the existing conversation thread will send to A's old number. However, texting group ABC by creating a new message and selecting the group from contacts will send to A's new number as expected.