Starting from Android 4.4, SMS/MMS blocker supports only on xposed enabled devices.
The section overviews how Blacklist works on to block incoming calls, SMS and MMS.
How Blacklist works
Every phone comes with a SIM card and a phone number. When a phone call is placed, the phone (caller) first talks to a mobile operator (ex. Verizon) and requests a call to another phone number (a receiver). The mobile operator check caller’s permission (ex. billing) and communicates to the receiver. On success, the receiver should receive an incoming call, receiver’s phone starts to ring. In this process, the mobile operator uses caller’s phone number and receiver’s phone number as the identification, as known as Caller ID. This process is almost the same for sending a SMS or MMS messages.
When talks to the receiver, the mobile operator optionally send the caller’s number to receiver. If they do, the receiver phone uses it to look up caller’s name and show both number and caller’s name on screen. Otherwise, receiver will see “private number” or “withheld”.
Blacklist relies on Caller ID to decide block incoming calls/text or not. Technically speaking, Blacklist intercepts all incoming calls and texts, extracts Caller Id (or null in case private number) and block calls/texts if necessary. Therefore, users should define which Caller IDs are blocked.
There are four alternative methods to block an incoming calls. One work on some phone models, others works on other phone models. But the four can cover all existent phone models running on Android 2.1+. All are done in the receiver side (and not caller or mobile operator sides)
- Turn ringer off: Blacklist will turn the ringer off as soon as a call arrive
- End call: The call will hang up as soon as it arrives. The caller perceives as you press the “cancel” button on your phone. In some carrier, the caller may be redirected to voice mail and they may leave a voice mail. To avoid leaving a voice mail, you can enable the pick up then hang up mode in the settings. SMS will be come to Blacklist box rather than normal message box.
- Hang up then pick up: When a call is coming, Blacklist tries to pick the call up, then immediately hang up it. This option can avoid leaving voice mail. However, it may cost you money if your carrier charged you for incoming calls.
- Turn radio off: in some ROM, both three above methods does not work due to the manufacturer did a lot modification on the original Android kernel (released by Google). This alternative method may drop your radio (include wifi, bluetooth) for awhile.
For SMS and MMS, it suppresses messages (that matches what users define when setting up Blacklist) as soon as it arrives and prevents them to enter default message box.
How to work Blacklist
To block a Caller ID (or a phone number), users should define:
- which numbers they want to block
- which blocking method they want to apply
Next, users should select a blocking scenario:
- Block Blacklist (excluded exception list)
- Block all excluded exception list
- Block All
- Do not block