Many providers actually let you use Wi-Fi calling without any VPN, i.e. they don't arbitrarily restrict the set of allowed IPs that can connect to their Internet gateway.

> If I have troubles receiving SMSs from Germany to German number while in US, would wifi calling icrease the chances of receiving the said SMSs?

Probably not, unless your provider supports inbound and outbound SMS via ISM. If you have an iPhone, you can check whether yours does in Settings -> About -> Tap the name of your carrier. (If it lists "Voice and SMS", you might be good; if it's only "voice" or nothing at all, SMS will go over the visited network.)