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I know this works the other way around because I have my old 5 here in Germany right now and removing my U.S. sim card and putting in a pre-paid German sim card worked just fine.
I'm due for an upgrade and my phone is basically on its way out. I'm considering buying a new iPhone 6 here in Germany. When I'm back in the U.S., if I put my old U.S. (verizon) sim card in, will the phone work as usual?
As long as the phone you buy is factory unlocked (not a "no-contract" carrier phone), it should work. Same goes for phones bought in the United States: as long as it is unlocked, there shouldn't be a problem
One thing to keep in mind, is whether the models sold in Germany support LTE networks in the U.S. and on which carriers the phone can be used. It's most likely that the only major carriers you'll be able to use in the U.S. will be AT&T, Verizon and T-Mobile, among a few smaller carrier. See this link from Apple that explained which models can be used on which carriers.
An iPhone 6S from Germany will include all of the frequencies and types of service that a 6S bought at Verizon has. This includes CDMA, GSM, HSDPA, and LTE. Modern smartphones from Apple and Samsung will work anywhere and all Verizon smartphones are unlocked.
I have used my Verizon S6 Edge this summer with a 3 SIM in Ireland and a German SIM in Germany.
There is something odd about Apple smartphones activating in countries other than the one they are purchased in. My daughter is trying to activate her new German 6S on Verizon. Verizon said that the model she had would work, gave her a SIM and told her that the Apple servers were slow. The problem is that her husbands identical 6S will work for a few minutes after a re-boot and then it reads "no service." This has happened multiple times after re-boots.
You will be limited to AT&T, T-Mobile, and other carriers that use GSM. Neither Verizon nor Sprint permit you to bring your own handset so you will not be able to use them. That restriction also applies to MVNO networks that use their networks.
Looks like you'll be OK with LTE frequencies.
