Hi there,
In Switzerland, the standard validity period for telephone bill debt claims is 5 years. The creditor (the telephone company in your case) must either renew their debt collection claim within that time-frame or write the unpaid bill off as a bad debt.
If you want to clear the score by paying your old phone bill, consider visiting a customer service center of the phone company in question (Swisscom, Sunrise or Orange / Salt, for example) while you are in Switzerland to find out if you still have an open account with them and if so, whether you can settle it using a postal deposit slip or by making a bank transfer.
Consider checking your credit history
It is also a good idea to check whether your unpaid debt affected your ZEK credit history. Negative entries related to debt write-offs for contracted services are normally removed from your ZEK credit history after 5 years, but it would not hurt to check. You can apply for a credit report online here:
https://www.zek.ch/de-ch/kredit-leasingnehmer/eintrage-zu-meiner-person
If the debt resulted in a negative entry in your credit history, you must ask your creditor (the phone company, in your case) to contact the ZEK and have the negative entry removed. If they are not willing or able to do this, contact the ZEK and file a request yourself.
If you will be visiting the municipality in which you previously resided, consider visiting the municipal debt collection office and requesting a statement from the debt collection register to find out whether any pending debt collection claims exist.
Conclusion
Considering that you have not lived in Switzerland for 15 years, there is a good chance that any debt collection claims against you have expired and that possible negative entries in your ZEK credit history have been removed. However, taking the time to ensure that claims are settled and that your creditworthiness is back on track can be a good investment if you plan to return to Switzerland at any point in the future.
Best regards from Moneyguru
More on this topic
ZEK Swiss credit bureau explained
Swiss debt collection registers explained