One other thing to remember with call forwarding your primary voice line elsewhere is, Text Messages do not forward. So, you will miss any that were sent while Call Forwarded.

TGarza and others on this forum have a lot of experience with the IMS function for iPhones, and I can also testify about it's simplicity and reliable performance. I have personally used it for a few years while traveling, and it works quite well. It can be set up while in the US if you have access to two different lines on any carriers. I suggest you practice the setup a few times while home to establish a comfort level prior to travel. It can be left set for literally weeks without issue, and is one of the best features of a dual sim, dual standby iPhone. While traveling with T-Mobile as my primary voice line, IMS calls register as a WiFi Call in my account manager. These are "Free" of roaming charges. You also can call a US number anywhere in the world for free, as the function sees it as being two US users, both Stateside.