P.,
I have 5 children and can I offer you a bit of advice based on my personal experience?
Consistency is the key to well behaved children, even at 20 months, even if she's not doing anything that can harm her, she needs to be in the mode of "if Mom tells me, I need to do it". What I am suggesting is that EVERY time she is doing something that is undesired (even though sometimes they are so cute you have to bite your lip to keep from laughing at them, so you can keep that stern look, so they know your serious!) Just simply redirect her attention elsewhere to divert her from what you want her to stop doing, it doesn't have to be a power struggle, just get her interested in something else, it doesn't take that long either, sometimes just going into another room, or picking them up and singing a song to them. By the time you put them back down they have forgotten what they were doing right before they were distracted.
As far as your son bothering you when your on the phone, what I do is keep a shoebox full of little toys that they can only have when I'm on the phone. Usually I pick up cheap things for a quarter or so at garage sales and will change the contents of the box after EVERY phone call (with the majority of the things being stored in a larger box somewhere else). You can keep reusing the same things, but if the contents are a little different every time, it keeps it exciting for them. Then just keep the box handy. When your on the phone and being bothered, just hand him the box and usually by the time they get bored with it, your phone business is done.
Good luck to you, and God Bless.