It sounds to me like she is a very lonely person to call all the time. It also sounds like she thinks you are sitting around doing nothing but waiting on her to call. Your very busy with 3 small children! Don't let her make you crazy and disrupt your schedule.
To begin, don't answer the phone every time (get caller id). If she asks when you do pick up the phone one day tell her you have a new schedule with the kids and you aren't home as much or busy upstairs doing things with the kids.
I like the suggestion of getting pictures etc on the website. You can also record the kids voices and put that in emails or on the website too. If that helps her be less lonely thats a good thing and it doesn't interrupt your day.
You can be proactive with some things and perhaps help her call less. Have your 2 and 3 year old draw pictures and send them off to her in the mail. She will love that! Be sure to send her any updated photos, or post them on the website. Instead of her calling, have your husband and the kids call her on a particular evening or on the weekend.
If she doesn't come down to your place enough invite her down (not too often)! With three small children a trip to other people's house is a logistical nightmare. Let her know that with all the special baby gear etc it would be a better visit if they came down instead of you going up.