At the end of Terminator 2, the T-1000 uses a needle-like finger to try and get Sarah to call out. However, we know from earlier on in the film, when John was talking to the T-1000 over the phone, that the T-1000 can emulate voices. Why didn't it use this ability?
The T-1000 only seems to be able to emulate the voices of people that it's heard speaking. Up to that point in the theatrical version of the film, the only contact between Sarah and the T-1000 has been at the barrel of a gun.
Until Sarah says "F*ck you", he's not heard her before and therefore can't replicate her voice.
On top of that, I think it's reasonable to suspect that John and Sarah will have a pre-arranged code to help him identify whether she's been replicated.
You may wish to note that in the film's official novelisation, the reason for him attacking her was that he genuinely thought that she might accede to his demands, offering further confirmation to the theory that he can't replicate her voice yet
THUNK! A steel needle slammed through her shoulder, pinning her. The polymorphic killer cocked back its other hand. The index finger extended as a gleaming needle, toward her eye, angling to slash through her frontal lobe and up through her upper medulla. It said in a smooth, chilling voice, “Call to John. Now.” Sarah had survived one metal motherfucker, only to be skewered by another. She was terrified, but more than that, she was mad. She couldn’t stop herself from screaming through her gasps of pain, “Fuck you, asshole!”
Once it successfully concluded its request had been denied, the T-1000 went into termination mode and arched back to deliver the killing blow.
Out of universe, there's actually a deleted scene in the film in which the T-1000 locates various tapes from Sarah. This means that he should have been able to sample her voice before the "Call to John" scene. Obviously that makes no sense from a continuity point of view which is almost certainly why it was removed.
Quite a bit earlier in the film, during John's call to his house:
John: Something's wrong, she's never this nice.
The T-1000 has already been foiled once, because it does not have enough information to imitate the behavior of a person - at least not enough to fool anyone but a total stranger. Getting Sarah to call out to her son would be more likely to sound authentic and it would also not require her to be silent as an imitation would...
That the T-1000 does not really understand what a mother is willing to suffer for her child is hardly suprising. It is a robot after all, the poor thing never had a mother...
Seems to me that being able to copy a voice is not the same thing as knowing how to behave like someone in a certain situation. So the T1000 might be programmed to torture victims a target cares about as an even better way to attract a rescue attempt, when an actual hostage is available.
There is another very good reason for this:
Stealth attempts had failed, therefore the full frontal assault between the terminators. After apparently pinning down the old terminator model, only Sarah and her son are left.
The son is nimble, unhurt and trained in warfare, he could hide very well in this noisy, confusing labyrinth of the steel mill. His mother is armed, but hurt and the terminator knows that at least this human relationship has very strong bonds. The answer of Walt mentions that the freezing process already damaged the T-1000, he is not in full shape anymore, he knows it and a longer hunt may not be an option. He is also aware that Jon knows of his mimic abilities and will be on his watch.
So the best strategy is to use John's Mother as honeypot, he only demands to "Call John out". If John is looking, he will see both Terminator and Sarah, know that this is his real mother in real distress and (hopefully) try to save or sacrifice for her.
In addition to the previous answer, we may always answer one thing to this kind of question: Robots are not claimed to be perfect entities and can also fail.