Correct english?

[5131elainen]

I will phone them again tomorrow
I will phone those again tomorrow

[jno]

them. 'Those' usually has to have a word after it ('those books', 'those ones').

[Quizmonster]

Both may be correct. If you phone people and don't get through, you might say, "I will phone them again tomorrow"...meaning all of them.
But, if you have sorted out the phone-numbers into two piles, A and B, and you haven't managed to get through to any of the numbers in Pile B though you did manage to speak to the people in Pile A, you might say, "I will phone those again tomorrow"...meaning just the ones in Pile B.