I set up three different extensions on my freePBX and using a softphone (3CXPhone to be specific) to make test calls to each other. But somehow, only one extensions out of three can make call to another. All three, I believe, are set up as “friend” users. Anyone has a clue?
Sorry, no mindreaders here.
YOU will need to supply the clues . . . .
In no particulary order, I ask:-
What is your network topology?
Where are the phones in that toplogy?
Are they registering to asterisk?
How are your PNAT/routers configured?
What does the asterisk CLI show as you attempt these calls?
Have you RTFM yet? (it’s on both the right of your screen right now and also at the bottom, the clue is it says “Documentation” 
)