My problem is similar to what I had with my Galaxy S6 Edge running Android 6.0. Now the same problem is happening with my new Galaxy S8+ running Android 7.0. The problem is this:
When I travel internationally, I use local SIM cards. My calling is 2 way locally, and I do not want clients to have to call my US number. I turned off "assisted dialing" to prevent my phone from out-thinking me adding international dialing prefixes to my local numbers. All local numbers have been edited to the number only, not country code or international prefix (+). This prevents the phone from adding 011-886 and the number. I have already set the correct country "Taiwan".
Local SMS texts go back and forth without problems, however international texts are coming and going in separate threads. This was only an inconvenience, once I understood what was happening.
I got an MMS last night which I attempted to forward to another local phone. Nougat insisted on adding 011-886 in front of each number and caused the sending to fail. I even typed the local number in as the forward recepient. This did not fix the problem.
This is a problem in Nougat. The logic which allows calls and text to be made locally fails to be consistent for MMS and international texts.
This is Verizon Galaxy S8+ with a CHT SIM card, in operation in Taiwan.