I'm with your sister. Just block his number and be done with him, that way you'll not be bothered with fluff from a prick who told you he didn't want anything else to do with you.
He cut contact because he said he was going to try and find someone new. That would be enough for me to send him a "fk you." to his "how are you" and then block and delete him.
Stop being devastated and for goodness sakes don't let him bring you back to square one with your healing by trying to figure him out.
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Adding: What does it matter if its "normal behaviour" after a breakup or not anyway? If you MUST return his text then next time he contacts you and before you block and delete him (or tell him "fk you" lol) ask HIM why he is calling you after he told you that he "didn't want any contact, not even as friends." Perhaps he'll actually just tell you and then you can satisfy your curiosity and immediately after his answer you can block and delete him if he comes up with a reason that isn't "I made a mistake and I want you back."
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion