Just keep it simple, address her by name and make sure that you say who you are if she does not have your number stored. Then try something like:
“Hi, _____. How is your day going?”
“Hope all is well. Would it be ok if I call you this evening to catch up?”
“Hey how have you been?”
All of those are non-threatening enough and I have used those hundreds of times.
Once you understand her humor, you can add something else.
Or just ask about something you know she has been up to.
or you can try Would you like to have coffee sometime? Keep it simple.