Hope my upping a two-year old (unsolved!) problem is okay here.
I have the same issue with a Verizon SIM-unlocked Samsung Galaxy S7. A friend of mine came from the US for a couple of weeks, installed a local SIM-card, but cannot send SMS messages. She is using Message+ app for messaging (I understand it came preinstalled from Verizon), and I failed to find the message center (aka SMS center or SMSC) number in the Message+ settings. Is there a way to change the SMSC so that she could not only receive messages, but send them, too, while away from Verizon? I understand Message+ is a Verizon-only app, but settings like SMSC cannot be hardcoded, there should be a way to change them. Otherwise my friend might have to switch from Message+ to some other messaging app.