Today when my daughter took a bath I wanted to go check on a thing, but of course couldn't since I was watching her. Of course it depends on the child, but in general at what age do you think you can leave the child to play in the tub (with not too much water) to go, say to fetch the phone or similar?
American Pediatrics Society recommends around the age of 6 before they are allowed to bathe alone; however, a quick google search will tell you that many parents start around the age of 4. However, usually parents who leave their kids alone make sure that they have no music on, and can hear the child clearly wherever they are. They also check in on the child every few minutes. As a parent, I am sure you'd agree that if the kid is making noise, it's probably fine. I wouldn't recommend going to answer the phone - it is quite distracting and you can "forget" about how quickly the time passes or fail to realize that your child is no longer making noise.
Also, note, that children are quite capable of turning the water tap on (unless yours is really high or something). They might not know how to stop it or be too scared (suppose if they turned on scalding hot water).
As a personal story: my mother left our 14-month old daughter in the bathroom alone. My mother had just finished bathing her and clothing her, when she realized that she had forgotten her diaper. She went to get her diaper - and did nothing else - to come back to find that our daughter had locked herself in the bathroom. Our bathroom is quite small, and the vanity drawers, when open, prevent the bathroom door from being opened. Our daughter had closed the bathroom door (she does that sometimes) and then proceeded to open the vanity drawers.
Our daughter freaked out because she could not get out anymore, and all we could see of her was from a tiny little crack. She is well aware on how to close the drawers (on a normal day anyway), but she was scared enough that she just stood there and did nothing. It took us a full 15 minutes to make our way in to get her out. Had she been old enough to get in and out of the tub, or to turn the water on, she surely would've drowned or seriously injured herself.
Kids do the darnest things and get into the strangest messes. Don't test them. No phone call, not even the one to tell you that you've won a million dollars, is worth your child.
Easy answer, while practicing to be in the bathroom alone, step out for a few minutes to take care of something and have the child sing the ABCs at the top of his/her lungs. Lots of fun for the kids, and you know he/she is still breathing.