How can I use my Android phone as a GSM Gateway?

 Nowadays, using Android phones as SIP clients is not a big deal. It merely has to be connected to an Asterisk server through 3G or WiFi as an extension. Any outbound calls will be routed through your Asterisk server, which will act as a VoIP gateway. Inbound calls may necessitate the use of a DID, a VoIP phone line that is typically inexpensive.

With quad core and octacore CPUs, Android phones have grown quite powerful. So why not put their might to good use? Why not install the VoIP gateway on the smartphone instead of the middleman, Asterisk? Installing Asterisk or Free SWITCH on an Android machine would be a fascinating experiment. It would turn the Android phone into a call center or PBX on the go. Using the Asterisk-enabled Android phone, other phones can join as sip clients. This would allow outbound calls to be routed through the Android device instead of through a middle application gateway. The Android phone will function as a VoIP gateway.

Let's see what we can do to make this happen and what the obstacles are:

First Challenge :

First and foremost, if we examine the Android software at a high level of abstraction, we can see that it lacks any high-level APIs for calling and controlling media. Playing recorded files on a voice call is an example of media control. This is stated in the official Android documentation. According to the datasheet, "The audio data can only be played back on a conventional output device. Currently, that is a Bluetooth headset or a speaker on a mobile device. During a call, you cannot play sound files in the discussion audio "..

Second Challenge :

The Android RIL's design is the second stumbling block (Radio Interface Layer).RIL is a layer that connects Android telephony services (android. telephony package) to radio hardware. The figure below clearly shows that RIL sends entire control of the call to the Linux library's Radio Daemon (rild), which subsequently passes the control to the vendor RIL.As a result, we are unable to control our call through the Android app. To alter our call, we'll need access to Radio Daemon and vendor RIL source code.

Third Challenge :

The vendor RIL is not open source, which is the third roadblock to making Android work as a GSM Gateway. Every vendor has their own RIL library that interacts with Radio Daemon (rild). Radio Daemon is a service that runs in the background on a specified port and forwards all incoming calls to the vendor RIL. Any incoming call is then captured and placed on the radio hardware by the vendor RIL library (system/libs/liberal-vendor.so).

 Conclusion

In its current state, Android cannot be used as a GSM Gateway. Even after installing a custom ROM, they are still reliant on RIL's proprietary firmware. Because the source code is available for the open-source community to use, hurdles 1 and 2 can be removed. The hardware vendors, on the other hand, are responsible for hurdle 3.The code for device drivers is rarely made public by hardware makers.

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