# Black and white lists
1. Go to **System** → **System file custmization**.
System file customization menu
2. Open the **extensions.conf** configuration file for editing.
extensions.conf
3. Set the "**Add to end of file**" mode and paste the following code:
```php
[add-trim-prefix-clid-custom]
exten => _X!,1,NoOp(...)
; Blacklist of numbers. The call will be completed..
same => n,ExecIf($["${CALLERID(num)}" == "491771789427"]?Hangup())
same => n,ExecIf($["${CALLERID(num)}" == "4915735987904"]?Hangup())
same => n,ExecIf($["${CALLERID(num)}" == "4915735996685"]?Hangup())
same => n,return
```
Code for extensions.conf
The whitelist of numbers sometimes needs to be described for specific providers:
```php
[Provider-ID-incoming-custom]
exten => _X!,1,NoOp(...)
; Whitelist of numbers.
same => n,ExecIf($["${CALLERID(num)}" == "491771789427"]?return)
same => n,ExecIf($["${CALLERID(num)}" == "4915735996685"]?return)
same => n,ExecIf($["${CALLERID(num)}" == "4915735987904"]?return)
same => n,Hangup()
```
* **PROVIDER-ID** - the value that you can find in the address bar at the time of provider configuration
