With sipgate team you can easily block unwanted calls to your account.
When you're logged in to your account please navigate to the account's inbox "My sipgate". After selecting the unwanted call or calls, you can find the blacklist/spam option in the More actions... drop-down menu.
Please select Mark as spam to block future calls from that number. With Remove from spam list the number can be unblocked again.
After selecting the unwanted call(s) and following the spam block dialog, any calls from the blocked number are marked with a Spam label and appear in the Spam section of your inbox. Calls from blocked numbers won't ring any User on your account.
To unblock a phone number again, please find the belonging call in your spam folder and select Remove from spam list. Calls from that number will then ring users on your account again.
Your phone or device may also have call screening and blacklisting options available.
You can also register your number with the TPS:
The Telephone Preference Service (TPS) is the official, freely available UK central opt out register on which you can record your preference not to receive unsolicited sales or marketing calls:
As TPS registration only prevents marketing calls, organisations will still be able to call you for the purposes of genuine market research.