As a hairstylist, the times I find it rude are when I need to communicate to the client, when her phone conversation is slowing my time making me late for other people (yes, it happens very frequently), when they constantly put the phone to their ear when I am trying to work on that side of the head, when the nod and laugh and such when I am working on them and they need to be still, when they are loud and obnoxious, speaking crudely and using obscene language bothering other guests as well as myself, (yes, I have heard it all!).
So, for nail technicians, while they don't have quite as many issues, so long as the conversation is brief, not obnoxious, and the you aren't holding them up, then I think it is okay.
Holding them up would be like if they ask you to have a seat, or to get up or move positions, or ask you for your nail color choice, and they have to wait several minutes for an answer or you to respond b/c you are too busy on the phone. Frustrating, especially when other clients are waiting.
It is rude when setting appointments, arriving to sign in and being introduced to the stylist/technician, checking out/paying when you are on the phone. Once you are seated is a better time.
But, it doesn't sound like you did any of that, I think you were fine and shouldn't worry about it since you were pretty discreet. Especially since the people didn't really speak English much. Really, it's sometimes hard to focus on doing hair/nails and the like when the person is talking to me the whole time. While our clients in teh salon our getting pedicures, we usually bring them something to drink and a few magazines so they can relax and occupy themselves.