..To be quite honest. You are being too soft and letting it go on too long.
I know it's difficult to adjust to handling domestic workers (I also found it uncomfortable when I first arrived here).
You need to tell her very straight. If there are 2 others in the apartment, get them there to give you moral strength. Ask her if she is happy working for you. If there is anything bothering her. Ask her is she thinks you are a fair boss. Listen to her and stay calm.. knod! Smile too!
Innocently tell her you are slightly concerned. Ask her who is calling her so frequently.
....then when she has finished, tell her very calmly (do not raise you voice or get emotional) that you are not happy. That you had given the situation a lot of consideration and rather than create any bad feeling you 3 have decided to explain how you feel things should be conducted. Make a list before-hand. Stick to it. Do not discuss it. It is not open to discussion - discuss it before-hand with your 2 friends. Give a solid, united face.. Agree that none of you will question the points in front of her. Save the toughest points to last.. Tell her straight. The handphone is to turned off during working hours. That calls are not work related and you are not paying her to chat with her friends. She is free to call during her break (tell her when that is) and during lunch times and her other free times.
I know this sounds rather harsh and unfriendly, but believe me.. ..she is taking advantage of you and you need to show her it's not going to be tolerated.
BUT, this is Asia so you must appear to be firm but fair. Stick to your guns totally and tell her you'll see how things go.
I have had serval maids.. I treated them all like friends. Big mistake! I came home once to find them having a party once. I pay a better than average salary and give occasional perks, but I also make sure it's appreciated now - rather than taken for granted.
Good luck!