Voip configuration

[hadi57]

hi
i need to configure 3 boxes of pfsense in 3 different geographic locations as follows:
1- location A have internet users and some can have access to use phone calls
2- location B same as A
3- location C same as A
4 - how is it possible for A, B & C to communicate with each other using the internet?
i am planing to practice this next week

help really appreciated
hadi57