My ex called me, what should I do?

My ex called me and I went into a shock and just stared at my phone while it was ringing.
we both liked each other a lot but I ended things between us because I felt like it was getting too toxic from both sides and I was too afraid to get hurt. We both have fear of commitment. I used to feel we were soulmates just both did not know how to adjust to each other.
He just called me after one month of not speaking. So I wrote him a message with why he called, he read it but did not respond to it.
should I call him back or just wait it out 2 days and then call him?
My ex called me, what should I do?
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