Do you have home wifi? Just turn on wifi calling and problem solved.
Settings / Phone / Wi-Fi Calling
Turn on Wi-Fi Calling on this iPhone
You will need to enter an emergency address. Use your home address since that is when you are likely to be on wifi calling
While you are there, turn on Prefer Wi-Fi while roaming - this will let you use wifi calling overseas which allows you to call back to US with no International charges.