Sonim devices have varying by model a lot of different custom buttons:

PTT_KEY

com.sonim.intent.action.PTT_KEY_DOWN
com.sonim.intent.action.PTT_KEY_UP

YELLOW_KEY

com.sonim.intent.action.YELLOW_KEY_DOWN
com.sonim.intent.action.YELLOW_KEY_UP

SOS_KEY

com.sonim.intent.action.SOS_KEY_DOWN
com.sonim.intent.action.SOS_KEY_UP

GREEN_KEY

com.sonim.intent.action.GREEN_KEY_DOWN
com.sonim.intent.action.GREEN_KEY_UP

Registering the buttons

To receive those intents you will have to assign the buttons to your app in the Phone-Settings. Sonim has a possibilty to auto-register the buttons to the App when it is installed. In order to do that you will have to contact them and get a package-specific key to include in your Manifest like this:

<meta-data
    android:name="app_key_green_data"
    android:value="your-key-here" />