Most modern telephone handsets and VoIP phones have options to block anonymous callers and to block or screen particular callers.
If you're using an ATA adaptor or DECT VOIP base station choose handsets with anonymous call rejection, call screening or blacklisting options and Do Not Disturb options. Some also allow blacklisting of ranges (e.g 084 or 087 numbers).
With the sipgate team service unwanted calls can be blacklisted from the event list.
If you're receiving calls from IP addresses, incomplete or invalid numbers and these calls are not displayed in your sipgate account's Event list, click here for help & information.
Please inform unwanted callers that you do not wish to be contacted again. Do not engage unwanted callers in further conversation and try to end the call as quickly as possible. Engaging the caller in conversation may only lead to even more unwanted calls.
You can also register your number with the TPS:
The Telephone Preference Service (TPS) is a freely available service. It is the official UK central opt out register on which you can record your preference not to receive unsolicited sales or marketing calls. It is a legal requirement that all UK organisations (including charities, voluntary organisations and political parties) do not make such calls to numbers registered on the TPS unless they have your consent to do so.
As TPS registration only prevents marketing calls, organisations will still be able to call you for the purposes of genuine market research.
Snom IP Deskphones:
With Snom Phones callers and Contact's can be easily added to a deny list.
To block anonymous calls add the following contact in the Directory menu of your phone:
Contact Type: Deny List
Outgoing Identity: Active
Gigaset DECT Handsets:
Many Gigaset DECT Handsets support "Anonymous Call Silencing".
Some Gigaset handsets also come with blacklists to selectively screen or block specific numbers.
Yealink IP Phones offer anonymous call rejection and blacklisting options.
Grandtream adaptors and deskphones support Anonymous Call Rejection.
Anonymous calls can be blocked in the device settings. Using 'Selective Call Forwarding' specific numbers can be blocked.
Panasonic KX series IP phones support options to reject anonymous calls and specific phone numbers.
Recent Panasonic DECT handsets offer Nuisance Call Block options to block specific callers and anonymous calls, while older handsets often offer some Call Barring options.
Panasonic Faq: Blocking Calls
BT Home Phones offer call blocking as standard with a wide range of options available including Do Not Disturb.