The callers are Vietnamese girls trying to steal your identity. If you call them back, then they would hang up. Thus, they know that you exist.
The next step that they would do is to forward all your mails to one of their mail addresses. Here are two of their addresses:
9851 BOLSA AVE, SPC #163, WESTMINSTER, CA 92683
8742 STANFORD AVE, APT 4, GARDEN GROVE, CA 92841
Both addresses are in Vietnamese community.
Once they receive your forwarded mails, they would try whatever they can to gather much information about you from your mails, such as credit card numbers. Then they would go shopping using your information or sell your information to others.