You won't be able to transfer your number to a German network. I know that sounds sort-of obvious, but it took me a long time to confirm that. Even if you use T-Mobile in the US and T-Mobile in Germany, +1 numbers have to stay in +1 networks, +49 in +49, and so on.

If you're like me, you've used the same number for the past 15 years. Separating from it would be a nightmare (all of your accounts are tied to it too!). US carriers don't have plans for expats, but you can't pay regular US carrier rates while living abroad because that's too expensive (US carriers are generally overpriced anyways), so you definitely need a domestic number.

Here's what you can do: Google Voice allows you to port your number to their service for a one time fee of $20. That means unlimited domestic (US) calling and texting for $20 once. Yeah, holy shit. Basically the catch is you don't actually get service with this. You'll still need a SIM card that can connect to the network. You can choose to forward your Google Voice calls/SMS to your new number, or you can install the Google Voice app and use it through Data or WiFi (quality is great). See here for some technical implementation, but keep in mind: I chose not to forward my US calls/texts to my German number. That would result in international fees. Instead, I use my German number to use cellular data (or WiFi) to use Google Voice for free.

When it comes to getting a German number, you basically have three options:

Telekom (T-Mobile): Best coverage, most expensive Vodaphone: Good coverage, decently priced O2: Kinda-shitty coverage, cheap

All of these have two options: Buy an expensive 30-day SIM card (which can be a pain to manage and if you fuck it up and have to buy a new one, you get a new number), or sign up for a 2-year contract. Both are shit options. BUT, you can also use resellers which use a combination of networks to get a really good price for decent coverage. A great website for shopping these is Check24. I ended up going with DeutschlandSIM which gives me a monthly contract, unlimited talk/text and 3.5 GB/mo data in all of the EU (Switzerland is not in the EU) for $22/mo. Boom. Downside is that it's all in German and their customer service is shit but whatever.

When it comes to Europeans, everyone uses WhatsApp. It took me a while to figure out why this is, but I digress because it's interesting:

Keep in mind the EU is relatively new (1993). While cell phones were becoming popular, you still didn't have complete cooperation between countries, so, if you live in Germany and make a French friend, you have to pay international rates to keep in contact with them. That's absurd. So when WhatsApp came around which offered calling and messaging over data (not bound by call/SMS rates), it gained quick adoption. Meanwhile in the US, everyone lives in the same country, so why the hell would we use an app when we can just send an SMS? See, isn't that cool? I mean, nerdy, yes, but cool. Right!?

So anyways, now the EU is strongly bonded and those international rates basically don't exist (or are regulated and kept low), so everyone could move back to SMS/calling (well, except Switzerland), but everyone is so used to WhatsApp that I actually had a coworker tell me today: "You know, you're the only person who has sent me an SMS in a long time".

So, download WhatsApp and make sure you have adequate data on your phone plan.

Sidenote: Your WhatsApp "ID" is a phone number, so I registered it with my US number. When you change SIM cards, you change numbers, and WhatsApp will ask if you want to update your WhatsApp number. Don't do this! This will change your WhatsApp number and all the people you met in Europe will lose your contact. Just choose to keep the existing number in WhatsApp.

In General: I use the Google voice app (US number) for my US friends / family / US accounts / DE accounts, WhatsApp (still on my US number) for European friends, and iMessage (DE number) for European friends that don't have WhatsApp or German accounts that require a German phone number.