This has actually happened with me. But it was my mom.
My mom was 19 when she had me and single so I lived with my grandparents from birth to 5 until she got her act together. So she didn't really go through the baby/toddler/kid stage with me.
When I had my 2 kids, my mom would call me and would get pissy if I didn't answer the phone every time she called. The kicker was though, when I did answer it and we'd be chatting and one of the kids was getting into something and I would yell over at them, my mom would immediately say something like, "ok, you can't talk so I'll let you go" and she'd hang up on me before I could even say something!!! It was so rude! Like what does she think I can do, let my kid run outside and just keep talking to her and ignore my kids??!!!
So finally after this happened a few times, and she did it again, I called her right back and was pretty firm with her. I said "mom, I try to talk to you when you call but I can't just sit there with undivided attention when I've got little kids I have to continually watch. You need to understand that they are my priority when I'm home alone with them and have to keep them safe. I love you and would like to be able to just sit and chat with you but I can't always do that. Please understand my life is not the same as it was when I was single. I have a husband and kids and I have to juggle everything around, including my phone calls with you. Don't get angry with me but instead understand I have little kids that I have to watch at all times." I think she mostly got it after that and was better.
Maybe you need to do this with your friend. If you tell her you have 15 minutes and she thinks that isn't long enough, then take that opportunity to explain why you can't just lay on the couch for hours and chat like you wish you could. Tell her you were just about to start the laundry, or dinner, or a work project, or dusting, etc, but you can put that off for 15 min so you can get caught up. Then she knows at least you are planning on doing something so she knows you won't be on that long. Then after the 15 minutes is up, say, I better get going so I can get my stuff done but I'm glad we could chat. Then she knows you appreciate her calls and are not just trying to get rid of her.
You mentioned in your last post that you work from home. If you are clicking "like" on Facebook all day and playing games, and commenting on stuff, other people can see that. If she "knows" you are home but you have time to sit on Facebook and waste time, she probably figures you have a lot of free time (since you clearly aren't working) and wants to chat with you since she's home during the day too.
So I think the easiest thing is to just tell her you are frustrated because you feel like she gets upset when you can't talk every time she calls. She'll either understand or she won't and if not, then you'll have to figure out what to do about the friendship. Good luck.