I think cellphones is very useful. Is this OK?-- No. Cellphones (plural subject of clause)...are (plural verb of clause).
Did someone/anyone throw something down the hole? Which is better and why?-- They are often used interchangeably by native speakers, but 'someone' sugges
Yes, 'using' is a non-count concept, so the verb would now be 'is'-- but the sentence you have created is terrible: "Using...is useful'?? Stick with the original.