I am working on a project that connects two user using opentok(tokbox now). It allows the users to be able to chat, do voice/video calls. But now we want that some admin can call another number even when they don't have my app installed.I want the admins to be able to call someone directly from the app and they shouldn't use their own phone.

Some example for this type of working is textnow, 2nd line, and much more apps like this.

So what i am asking here is that which third party services i can use in order to achieve this task.