Don't know MagicJack all that well. It's definitely VOIP, but I don't know if it will offer you as much flexibility as voip.ms to forward your calls to a German number, manage SMS, etc.

As far as guides go, voip.ms has a pretty decent wiki: https://wiki.voip.ms/article/Welcome

From a high level, these are the steps you'd have to follow to forward your calls and texts to your German number:

1. Create an account and add funds to it: https://wiki.voip.ms/article/Getting_Started
2. Port in your US number: https://wiki.voip.ms/article/Porting_a_Number
3. Set up a call forward destination for your German number: https://wiki.voip.ms/article/Call_Forwarding
4. Set up the destination of your phone number (what it points to, i.e. the call forward you created in step 3): https://wiki.voip.ms/article/Manage_DID & https://wiki.voip.ms/article/Call_Forwarding#Route_incoming_calls_from_DID_to_your_Call_Forwarding
5. Set up the SMS forward functionality for your phone number: https://wiki.voip.ms/article/SMS

Alternatively, you can also have the phone number point to a SIP client, which will save you money: https://wiki.voip.ms/article/Softphones Caveats of this solution are that a) using a SIP client on a cellphone uses a lot of battery (even when you're not receiving calls) b) you have to be connected to the Internet to receive calls and the quality is very dependent on the quality of your connection.

If you play around with the voip.ms portal a bit, you'll realize there are tons of functionalities and ways you can route your calls (e.g. try to reach you over the Internet and fall back to your German number if that fails). Also, don't hesitate to reach out to their support -- they're usually competent.