Moving to Germany from USA to live for at least 3 years to study. How can I still use my phone to text/call/FT Americans while I’m there?

International roaming is very expensive and it's not worth it if you'll be staying for so many year in Germany. I suggest you go to Germany and get a prepaid SIM which you can use to with your iPhone. Your iPhone needs to be unlocked though with which I can help.

Email : yanastorff@yahoo.com

You can do so exactly as you would in the USA, provided that your call plan has “roaming” enabled for both calls and data.

But roaming internationally costs a fortune: you also have to pay not just for outgoing calls, SMS, and MMS at international rates, but you have to pay the international cost portion of incoming communications from the USA.

So if your phone is a GSM model and is compatible with the frequencies used in Germany (such as all current iPhones), then make sure before leaving the USA that your phone is unlocked to accept any SIM and then buy and install a local German prepaid credit SIM. That will also get you a German mobile phone number and under current EU rules you can then use that in any EU country for the same cost as if you were still in Germany.

Whenever you wish to phone home just use whichever SIM is offering the cheapest rates to the USA. Or if you have an iPhone and are calling another iPhone user, use the free FaceTime (audio or video) or iMessaging.

You can keep your number and "roam" but the charges can get out of hand. Check with your carrier about international roaming plans, or you can get a locally-issued SIM over there and swap in and out as needed.

"About cellular data roaming options for your iPhone and iPad"

https://support.apple.com/en-us/HT201643