I understand but I'm not sure its a red flag! If she stuck around the house with the kids I don't see the big deal and I don't see how it relates to her disappearance.
Agree that there's no obvious connection. If she drove with the kids, I could see daughter insisting on one, but we don't know that she did. She very well may not have, as it's tough to deal with three very small kids with no help and she'd have needed three car seats installed in her car.
If she never took the kids out in her car, I don't see why she'd need a cell phone. The landline would be sufficient at the house and surrounding area and even if she went for walks and a kid fell and got hurt, she probably wasn't too far from home or worst case, a neighbor's phone.
Sandra apparently didn't have an income beyond whatever she got from her daughter. She also apparently didn't use/need the internet and it doesn't sound like she got many phone calls. She may not have felt the expense of another phone line in addition to the landline was needed under the circumstances.