This is interesting, it seems to be developing a subtext of SAHM versus working mum. As some of the other posters said, who knows what the call was. I am a working mum...therefore if you see me on the phone when I'm with my daughter, you may think I'm doing 'work'. Sometimes...yes I am! As another poster said, I too work flexibly so I can pick her up from school and spend many hours happily free from interruption, but other times she knows I may have a work call.
For the last few months, I have been supporting a friend who has severe depression. She has been very bad on occasions. I will take a call from her at the moment, even if I'm on the loo!!! You may sit in judgement of me, if I do this in front of my child. That's OK...that's your judgement, it doesn't really affect me, and I'm sorry if my behaviour would make you depressed. That's sad that your mood would be affected by my behaviour, for which you have no underlying context to explain.
I have a lovely, understanding relationship with my daughter...sometimes she knows she has all my attention and the phone is pretty much off limits during bedtime, much to the irritation of many of my friends. But not the one who needs extra support right now. And I have explained that to my daughter too. She knows how important she is to me, but she understands the care and support we can all give to others.
Be calm, love your own children, and be depressed if you see a child being hit, neglected, ill or wanting of love. Be careful of imagining circumstances that are not your own...maybe one day they could be your circumstanes.