When you google xda developers sonim xp3 there is a thread on rooting it and being able to install apps without adb i couldnt figure it out but maybe someone else can root one of the versions sprint or us cellular?

Random This app goes on without ADB. Maybe someone can take a look at it and see why it is able to go on without ADB.

To ensure that Zello will install and work properly, you must use the most recently signed .apk file with Sonim's signature from our website. The link to this file is here.

But what does that mean? And i dont know if its possible to do that to a regular apk

Rooting it maybe the only option

Random

Random But what does that mean?

What are you referring to?

Signing an app with sonims signature

Random See this.

Apperaatly its possible if Zello was able to do it.

Zello probably worked with sonim if I remember clearly it did not even work until one of the updates

TryingToBeHelpful they don't do it sonim does it!