Ok, I think I found the solution from this forum to my 2nd question:
https://forum.sailfishos.org/t/quick-access-for-covid-certificate/7962/4
The call forwarding thing however does not work completely as the string after the tel-part ignores all symbols which are needed when using the call forwarding. (**21*number# to enable forwarding and ##21# to disable it)
EDIT: Also even if one makes a simple link to an ordinary phone number without any symbols, it hangs to a spinning wheel around phone-symbol.