If you find out you have another operator without having asked for it?

An operator may count you among his customers only when he has your agreement and if that agreement has been provided to you on paper or electronically (e-mail); therefore an agreement made over the phone will have to be confirmed in writing. Always ask your operator to provide you with a written copy of the agreement made.  In this confirmation letter it should say that you have 14 days to reconsider and to annul the migration. If no time to consider is mentioned in the confirmation letter, by law you have three months to reconsider.

You need to inform the operator who has migrated you without your consent that you do not agree with his migrating you.

You can let your former operator know that you have been migrated without your consent and ask to return to him.

Next, you can submit a complaint to the Ombudsman Service:

Telecom Mediation Service
Boulevard Roi Albert II 8 boîte 3
1000 Brussels

Telephone: 02 223 09 09
Fax: 02 219 86 59

Website: www.ombudsmantelecom.be 
e-mail : klachten@ombudsmantelecom.be 

There are sanctions for operators who violate the legal obligations regarding the customer's consent.