First, I need to decode your situation. I am assuming followings.

1. You currently live in Switzerland.
2. You currently use Wifi in Switzerland on Voip whose phone connection layer/phone number is provided by www.switzernet.com.
3. You want to use Switzernet connection using your Android phone in the US and European countries other than Switzerland.

The answer is: it depends. Probably works.

These are usual issues.

1. Connection speed. Read the FAQ section of your carrier switzernet.com. It says 100kbps up and down. Now, my experience is that Wifi in Switzerland usually exceeds this requirement. Whether the connection meets this requirement or not depends on the Wifi provider: apartment, hotel, in other European countries or in the US. If you get stuck in a room away and beyond signal shielding objects, you are out of luck, at least in your room, even if the speed at the router is fast enough. Sometimes I am only provided below 30kbps connections and even that was available in the restroom where the entrance fire door did not block the signal.

2. The Wifi providers can place restrictive traffic filters. They either do this intentionally or unknowingly. Either case, if your Voip falls in that bucket, you are out of luck.

If the connection is important with you, have a fallback plan if you cannot receive calls.