I doubt that knowing a person coming after him and hunt him down, he would still appear with the same appearance (i.e no disguise) on the street, where everyone can see and recognize him. If there was a random BO member (doesn't have to be Bourbon) hanging around then he's done (and so will Kir). The same argument for his first and second appearances (where many Org. members saw him).
So far there have been many possible explanations for this (the following are just examples):
1. It was just a coincidence. One of them really did answer Vermouth's call. The other two answered different calls from different people (I don't know who though), but at the same time. Gosho obviously cannot show one of them receive the call, as it would be too obvious.
2. Only one of them really answered the call (from Vermouth). The other two (one or both) bugged that call, which explains why they rang at the same time. Although I find this rather unlikely.
3. One of them really answered the call (from Vermouth). The other two (one or both) make a call. Note that it only shows that Amuro's phone actually rang. Sera was using a different device, it was hard to tell whether she called or receive a call (the SFX was more like pressing sound). Okiya's *blink* effect was ambiguous, it doesn't necessarily mean that he was receiving a call (since he was sitting in a car, he might have actually heard the radio or something).
4. None of them received the call from Vermouth. Bourbon is someone else, Vermouth was there for different purpose. Gosho draws that just to trick us.
Amuro is shown most of the time to use right hand, and so is Sera: http://www.detectiveconanworld.com/wiki/User:Chekhov_MacGuffin/Handedness. About your wondering, answering phone calls and doing other simple actions may not always follow the handedness. (All of the 3 use opposite hand to answer the call btw).