Why would someone call you and then just do their work silently?

I've a very good friend. We share a lot of life views and talk a lot often. However, I never understand one aspect of his behavior—he sometimes calls me/ I call him and he talks a few words. Then he would go silent, just keep doing his work or go on his post-dinner walk but won't discontinue the call either. In person too sometimes he'll just tell me to sit beside him and then he keeps doing his work. Often when we're talking, at times we're just silently sitting or standing beside each other. Is it normal? He doesn't do it with anyone else.

Why would someone call you and then just do their work silently?
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