flaviam
Welcome to the community, Flaviam!
This is a user-to-user forum and therefore we do not have access to your customer data or contract details.
Since debt collection agencies are involved, it is best to handle this in writing. You can submit your formal written dispute directly via the official Sunrise contact form.
https://www.sunrise.ch/en/support/contact/contact-form
It is also helpful to send a written objection to the debt collection agency itself to contest the claim.
Cheerio, cosmo