So if you set Þórunn's nickname to be "thorin" Siri will be able to recognise it. Basically just go through your contacts and give them all nicknames that are English phonetic spellings of their real name.
Before that she was only "mamma" as first name but I had other "mamma zoe", "mamma ale", "mamma gio" and Siri always wanted to call "mamma zoe" when I said "Hey Siri call mamma".
After filling some fields of my mamma she started asking "Who would you like to call?" and started saying all mammas in my phone book, but when I said "mamma mamma" she started calling a random one or not getting it.
Now that I have my Mamma (first name) Mamma (middle name) Mamma (last name) of company Mamma nicknamed Mamma with email mamma@mamma it works... go figure!
"Call Þórunn mobile." "I don't have a Thorin." "Call Porun mobile" "I don't have a porun".