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?
The American Academy of Pediatrics (AAP) recommends the age of 4 before children are allowed to bathe alone; a quick Google search will tell you that many parents start around that age. 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 it 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 easily could have 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.
I guess I am from the old school. When I put a child in the water it's to wash them and get them out. If I have time for play I will put a toy in the water and if I need to do something, it will be something I can do in the bath room. I'm not willing to take a chance. Somethings we can not control when it comes to safety but what we can control we should do, especially those who can't do for them selves and depend on us. I was in a place where a 2 year old was put in a tub of water and the mother did leave the door open but had not checked on the child for about 2 min. I heard loud splash but still no mom. I went to the door of the bathroom and started talking to the 2 yr old thinking she would come. when she finally came out the room I told her she shouldn't leave a 2 yr old alone in the tub especially with that much water. She then looked in the bathroom and then said I put the water level at the height I do when I give him a bath, and it takes 4 min for him to drown. I will be able to hear him. I then said you should never say that out loud you can go to jail for a long time. I didn't laugh. so at this point I am so angry. Do I think she is a bad mom, no I don't but that was not nor should it be funny.
I think about it this way. If I worked at a daycare would you want me to leave your child in water unattended for any amount of time. NO not at all the day care would be shut down and it should. So where does the mind set come from to do this at your home? Does your child mean less to you?