OK, after reading some in the forums, I got the matvt 1.0.3 app working on my Sonim xp5s. If the select key is the same on the xp3plus, I think it might work. I do not have an xp3plus, so if any of you do, could you follow the steps I list and confirm whether this works or not? I first uninstalled the apps4flip mouse.
Then, I installed [Login to see the link] using adb.
Next I ran the following adb commands:
adb shell pm uninstall -k --user 0 com.lge.voicecommand
adb shell appops set com.android.cts.io.github.virresh.matvt SYSTEM_ALERT_WINDOW allow
adb shell settings put secure accessibility_enabled 1
adb shell settings put secure enabled_accessibility_services com.android.cts.io.github.virresh.matvt/com.android.cts.io.github.virresh.matvt.services.MouseEventService
adb shell am startservice com.android.cts.io.github.virresh.matvt/com.android.cts.io.github.virresh.matvt.services.MouseEventService
I got the standard "Error: Requires permission not exported from uid 10104" message from the last command, and ignored it.
I restarted my phone.
I opened the matvt app in my applications list.
I selected Override Activation Key
In the box underneath that, I typed 18 (for the # key), then selected SAVE.
Now, when I long press (for about 1 second) the # key, a mouse appears on my screen. It moves with the D-Pad, and the center D-Pad button does select things.