This sounds like a good example to practice SET. In my opinion you already mentioned everything that is important in your post:
Support:
I do want to chat and hear about her day
Empathy:
she's driving, or she's somewhere on her own and has free time.
What might be her underlying feelings here? How could you adress them according to SET?
Truth:
but since its the workday, I can't stay on the phone for half an hour or longer.
I encourage you to try formulating this in a way that would be suitable for you next call. Feel free to share your thoughts!