How to organize calls to landlines and mobiles through the application?

Question

Telephony

How to organize calls abroad (themselves in Belarus), more calls to Russia to landlines and mobiles.
For now, I imagine it like this:
1) employees have androids on which the application will be installed (for calls via the Internet).
2) there must be "centralization" (for example, login and password to connect to the server or a shared account).
3) I want the ability to restrict calls to certain country codes, etc.
4) the service must work both through corporate wifi and through the Internet. anywhere (this is in case of an option with the purchase of a server or other equipment).
PS> Maybe there is something similar for Skype, Viber, ...

Answer 1

Option 2.
1. Buy a ready-made virtual PBX, there are an uncountable number of operators on the Internet. Inside you can customize everything you need. This option is more expensive than the second one.
2. Put your PBX (asterisk / freeswitch), user account and rights do on it. You buy only a trunk for calls from operators. This option is cheaper than the first one.