Assume the following:
You add a contact (let's refer to them as "Person A") to your iPhone and add two separate numbers for them - one entry labeled "Home", and the other as "Work"
You open Messages.app, and compose a new message to Person A's Home phone number - this creates a new message thread, because up until this point you hadn't ever messaged them previously
Person A then replies to your message, but using the number that you have labeled as "Work"
Whenever the above scenario was encountered, I can recall that in older builds of iOS that two message threads would have been created - one for each number (in this case, Home and Work). Now, however, iOS appears to combine the conversations from both numbers into a single message thread because it recognises that despite being separate physical phone numbers, the contact is still the same person.
So - my question. Would somebody be able to clarify which version of iOS introduced the new behaviour described above?