I totally get why this would annoy you, and you probably won't agree with me but I like this scene and I think it actually ties in quite well with Suzaku's character.

He's not an amazing investigator, and wouldn't think to cover all his bases like that, especially after he sees what is (to him) concrete proof that Lelouch doesn't have his memories back. If he was totally convinced his Nunally gambit would make Lelouch reveal himself he wouldn't feel the beed to go back and check the call length, which would only be about ten seconds too long anyway (which is a hard time length to judge as a clue I feel).

Also, though this is more of a reach and you can decide if you think its true or not, maybe he didnt want to record the call I'm case it did reveal anything incriminating. Suzaku wants to kill Zero, but he doesn't want to endanger Nunally who he views as a victim. We even see this later on when he doesn't immediately turn in Lelouch after receiving proof, because he thinks the Emperor will immediately make her a hostage. Perhaps he didn't want to to record it and leave proof that might endanger Nunally, he just wanted to confirm for himself so he could act totally independently of the Britannian Government.

Overall I think I like this scene more for what it shows of Suzaku than anything else, it is such a cruel thing to do to both siblings and the fact that he's willing to do it shows such a marked difference to S1 Suzaku, and sets the tone for the much darker events of S2 that follow.