Do you mean wei 2 ?
When you are on the phone, wei2(喂）is the greeting for saying hello.
na4jiu4zhe4yang4 (那就这样）= I gotta go.(when you wanna politely tell the other that you are about to finish the talk )
Both a(啊）and O（喔）are used as interjection, it's all up to you to decide which one to use. They sound the same to me. Using the interjection at the end will show the other that you tend to have a soft tone to your friend.