The free Callfilter uses a continuously updated black list to block phone numbers. They come from dynamic lists of known spam callers. The list includes the phone numbers of call centres that use spurious sales pitches, call at inconvenient times (before 8 am or after 8 pm and at weekends) or do not comply with the Callnet.ch Code of Conduct. The code includes respecting directory listings marked with an asterisk to stop nuisance calls and display of the caller’s own number.
The list also contains numbers blocked personally by fixed network Callfilter users. Customers block around 8,800 numbers daily. These numbers are not added to the black list immediately; they are anonymised and used to continuously improve call filter effectiveness. Callfilter also checks a number of criteria before the relevant number is placed on the general Callfilter list.