Being a mother of two children, I can honestly say that I never would even answer the phone when they were in the bath.  I was always over cautious.  I let the phone ring out.  There's just times you don't talk on the phone as a mother.  It can always wait.  
Same here, my guy has a bath once a week and I would NEVER EVER leave the bath, if if I have to run and get a towel and the water is already out I have  real bad feeling like he would slip any second.
kids drown so easy and so fast.
We have a friend who put her 4 year old and the 2 year old in the bath, she goes down to answer the door, comes back up 10 min later or so - small kid is under water drowned - the other boy though he was making fun.
You don't fucking bounce back from that, that's your life gone!!