Hello Ryan The choice is "than". You use "than" to introduce the second item in a comparison: 1. I 'm bigger than her .
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MrPedanticBut the idiomatic meaning of the "...no sooner...than..." structure is that the second item in the comparison happened immediately after the first item. So here, the sentence means "he sat down, and the phone rang immediately".
MrP
Is it synonymous with "as soon as he had sat down..."