It an android application for supporting to GSM Modem (SMS) android app. Due to Google policy GSM modem app removed all SMS permissions and created new Helper tool open source in that support. Now you can write your own application to support that app for your work continuity.
Before installing this app you must install GSM modem (SMS) app in your device, It can be found on Google PlayStore or visit following link for further information and guide. http://sindhitutorials.com/blog/gsm-modem-free-sms-android-app/
This is for latest android version, download and install in your mobile and start sending and receiving messages. https://github.com/sadiqodho/GSM-Helper-Tool/blob/master/download-app/latest-helper-tool-app.apk
If you are not android developer and want to download built application then You will find app here on following directory. But this application for lastest android versions like oreo android version and above. https://github.com/sadiqodho/GSM-Helper-Tool/blob/master/download-app/app-debug-above-26.apk
If you are not android developer and want to download built application then You will find app here on following directory. But this application for older android versions like Nougat android version and below. https://github.com/sadiqodho/GSM-Helper-Tool/blob/master/download-app/app-debug-below-26.apk