I have an iPhone SE with iOS 13.2
Recently I noticed that no SMS messages arrive to one of my contacts (using an Android phone).
The contact has multiple phone numbers and I suspect the SMS is sent to a wrong one (not currently in use).
I think this started happening with the update to iOS 13.
No matter what I do, the messages don't arrive.
I had a similar problem before, because the Messages app does not show the used phone number, but only the contact name, so I was sending messages in thread A (the right contact, but wrong number) instead of thread B (right contact, right number).
But with iOS 13 it seems all threads of a contact are merged and I'm stuck with it.
If I try to start a new thread, select the contact and the correct number, I get transferred to the existing thread. Then if I type and send a message, it does not arrive.
Interesting detail: when I do the above (new message), when selecting the contact, by default the wrong phone number is selected. Apparently this number is used, no matter what I do.
One more detail: if I try to see which number of a contact was used, using his "simple" procedure: open thread, tap contact name on the top, tap "info", tap the ">" sign at the far right, then the "RECENT" mark is on the phone number that I last selected for sending SMS. But this seems just to be a GUI thing, as the message never arrives.
I don't have problems sending SMS message to other contacts (most of them have a single phone number).
I also have no problems receiving messages from this contact. They appear in the mentioned existing thread. I just can not answer.
Best Answer
Apparently this was fixed with one of the recent iOS updates.
Currently I have iOS 13.3 and this problem is gone.
Edit: "fixed" is an overstatement. I can now send a message that arrives by manually selecting the correct phone number. If I write another message in the same thread, it is again sent to the wrong number. The RECENT label, if inspected, is again on the same wrong number, as before the "fix".