I am from Australia and I have issues sometimes too running on GSM!. Things like failed texting and even calls failing because my phone thinks I'm dialling overseas. I resolved all of mine by changing all of my numbers to their country code extended file name. for example numbers in Australia look like this "0403045209" but I have now added my country code which is "+61" while using this code the first digit of the number needs to be dropped. In the end the number looks like this +61403045209. Everything now works perfectly fine.
If you are thinking to yourself oh crap now I need to go through and edit them all. The easiest solution is to go to your computer and log in to your gmail, up the top left you will see the "google" logo. Just underneath that you will see "gmail" click on this and change it to "contacts" - This might sound stupid me explaining it but it took me ages to figure this out lol.
I don't know if changing your numbers to their extended country code form will help, but it's worth a shot. For me, if I didn't do this, my phone thought I was dialling overseas. I think it is due to being Verizon, I dunno.
EDIT* I only needed to change "mobile numbers"
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