Blocked from getting a phone contract – no explanation given

I’ve been living in Switzerland for 5 years and moved to Zurich about a year ago from another canton. I’ve always paid my bills on time and never had any issues, but today a cellular provider refused to give me a long-term contract, saying something about me being blocked for credit, they cannot see on their system why, might be an unpaid bill but suggested I'd contact the relevant authority in this regard.

I’m confused since I don’t think I have any unpaid bills or past problems. Where should I check, and what could be causing this?