Thank you for the interesting question. I have looked into this quite a lot and I have come to some conclusions. However I don't have a definitive answer for you.
First of all: how did telephone numbers look in Stockholm in the 1930s? This is a tricky question. They could look quite a number of ways. During the first half of the decade it seems they had numbers three to six digits long. During the middle of the decade area codes were introduced in Sweden, so then telephone numbers could be up to nine digits long. The area code in Stockholm was 010.
However a lot of digitized business cards from 1930 that I have looked at do not have telephone numbers on them at all. Some business did have telephone numbers though, as you can see on the attached picture. It shows a business card from the city of Nyköping from the 1930s and it has a three digit number: "Rikstelefon [Kingdom telephone] 382".
You can find some information on The Swedish Post and Telecom Authority's web page. The historical information is in Swedish, but you can translate it via Google Translate.
For more information I recommend you contact the Stockholm city archives to see if they have any material and that you contact The Swedish Post and Telecom Authority (PTS = Post- och telestyrelsen). Hopefully they can give you a more detailed answer.