Friends
Friends are people on MFICore you have agreed to connect with. A friend can see when you're around and call you, and you can call them.
Adding and removing
Send a request by username or email. The other person gets a message and a notification, and the friendship starts when they accept. Declining is silent: they aren't told.
Removing a friend ends the friendship for both of you, and neither is notified. It does not block them; they can send a new request. To stop someone contacting you at all, block them from the Mailbox.
Calling
A friend call goes through a media server one of you has picked under Account → Profile → Voice & Video. If both of you have one, the caller's is used.