In my experience, wi-fi calling works as if on the home network, regardless of IP location/endpoint. So this would work similarly with an eSIM.

> 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?

I'm not up to date on the state of messaging infrastructure but it used to be the case that some providers would offer non-standard methods for sending messages over their network to intermediary providers. Rather than sending an SMS to a number, a business would ask the intermediary to send a message and the intermediary would use the non-standard method provided by the network provider. The non-standard methods work fine if you're connected to the network directly but if you're overseas that will not be the case and so you can't receive these non-standard messages. Don't quote me on any of that, though.

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.)

Alternatively, use a global eSIM purely for data access, then use your phone as a SIP client (or use something like Google Voice) for PSTN access, eschewing the mobile network entirely.

Yes, see: https://news.ycombinator.com/item?id=42768735

Are there even roaming costs at all when calling over wifi? Would make sense to me if there weren't, since you're not using the mobile infrastructure you would be paying for.

Then again I could also easily see telcos charging roaming anyway, just because they can.