Nothing is ever totally free. There has to be some strings attached.
Google Voice gives you a free USA number, but they only forward to other USA numbers. They used to have a free internet connection through Gizmo, but it was turned off.
Various VOIP services will give you a free or near-free number, but they will not forward to your real phone for free, only to VOIP endpoint (such as computer or another device that registers over the internet). Certainly not to mobile, because those are very expensive endpoints.
Something like Voxeo will give you a free phone number and may even proxy to international location, but only for a test of a new application you would be developing.
So, nothing is completely free. Here is what is probably your cheapest long term setup:
Buy a pay-per-use phone number. USA numbers can be as fee as a $1/month + per/minute cheap charges. Look at something like voip.ms or Vitelity.
Buy VOIP hardware that connects to your internet router at home and gets normal phone then plug into it. Sipura (now Cisco) used to have good cheapish devices or Fritz!box if you want fancy German one with (a lot of) room to grow.
Learn to configure this all to work (this is money-cheap, but time-expensive) part
Alternatively, just look for providers that advertise internet/VOIP phones. But, of course, they will not be free. If you want to go all the way into the rabbit hole in this direction, just start searching for "Direct Inward Dialing" (DID), that's the technical names for phone numbers when divorced from the rest of Telecom concepts.