Reason for beeping sound then drop call

[Windows Central Question]

Why do I get beeping sound then say lost connection when I am talking to my friend in the Caribbean but not when I talk to friend in Japan? She talks to others and has no problem and I talk to others and no problems just when we talk to each other very frustrating because this friend is who I mostly talk to and the reason I have Viber so many drop calls is making me so frustrated