Force country code on known numbers to send an SMS I've been looking on the internet for an answer, to no avail. I have a phone mostly used in France, I recently brought to the US with a US sim card. Unfortunately, most contacts were added with the default French format (ten-digit number starting by "06" or "07"). 

Unfortunately, here in the US, one would need to have the country code in front of the number (+336 instead of 06) for the text to go through the US network. And this is where Android shows its weaknesses. 

Indeed, even if you:


delete a complete SMS conversation
add the country code to the contact, and then change one digit at the end of the contact so the number associated to the conversation should be removed from the system
reboot the phone
wait 1 day for anything delayed to take effect
change the digit back into the real number, this time with country code
try to send a text by explicitly typing in the number (not the contact name) with +33


Then, it does not matter that you have done steps 1-5, Android will still "snap" your complete number with country code, to the default one he had (and still remembers somehow), which was without.

It now comes to the extent that I especially pay attention when writing a new contact, to always add the country code, since the first time the system sees the number, so that it can never dream of removing that country code by itself.

I have tried installing a third-party SMS app like Pulse SMS, but it uses Android's built in SMS contact book, so same "snap" problem.

What I would like instead, would be an SMS app that ignores the contact book, or recreates its own from the real contact book (not the image of the contact book stored in the SMS app), so that I can finally "force" the country code onto the SMS app (or more simply, that adding the country code in the address book would add it in the SMS app, which would be normal behavior), so the SMS can be sent, otherwise, it can't!

Does anyone know of such a low-level SMS app, or of a way to trick the built-in app to finally use the country code?

Thanks!
I confirm that the problem persists on a pixel 3a. But, it is kind of reversed. In my case it is a US number and the SMS messages are linked to the number with the country code. But, within the US the messages must be sent without the country code. I find the same as the OP, there is nothing I can do to prevent the country code from being added. Delete the SMS messages, delete the phone book entries. Android is smart enough to always link the number I enter with the old entry that included the country code, recalling it from oblivion. But, it is too dumb to know that the message must be sent without the country code.

Also the answers given are missing the point. There is nothing we can do to control whether the country code is added. You can delete the contacts or edit them any way you like, the original form of the number will always be retrieved, with the result that you cannot text that number.