The T-1000 had past experience of trying to imitate Janelle (John's foster parent).
It tried to pretend it was her when John rang to warn them.
But it took the wrong tone with John & he knew something was off.
So that attempt failed, and it got to
confirm that when it checked the dog's name.
That, added to the damage it had sustained, probably convinced it that imitating a woman that was even closer to John would be a failure.
Hence, next strategy, torture compliance out of Sarah (which ended just as poorly, in the short time it had access to her).
movies.stackexchange.com/users/566/Andrew Thompson
A possible reason is raised in a deleted scene:
The T-1000's mimicking skills are compromised after freezing, melting and cohering again.
From the IMDB FAQ : The T-1000's decreasing efficacy is explained in a few scenes cut from the theatrical version of the film but are able to be viewed as part of the director's cut or on most versions of the DVD as a deleted scene.
After being frozen in the liquid nitrogen and then shattered, the T-1000 starts to show evidence that it may be malfunctioning.
As it is walking, its feet, up to about the knee, involuntarily imitate the diamond plate flooring it walks across and its hand bonds with and imitates a yellow-striped railing.
The T-1000 itself seems confused by this but is unable to stop it.
Later, when it calls John in Sarah's voice, the glitch occurs again:
John glances down, and there's a quick shot of the T-1000 imitating the grating its standing on right before Sarah shoots it the 1st time.
In the theatrical version, the only evidence of the glitch is the slight flutter that you see after it crushes Arnold's arm in the gear.
This may explain why it moves much more slowly during the final conflict, which allows Sarah to escape while it is dealing with the T-800. TVTropes raises a couple of other valid points, like: T-1000 is unable to replicate emotions that well; T-1000 wanted to put Sarah at risk so John would come to her aid, since this happened before.
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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 which means that 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.
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It can reproduce her voice mechanically, but cannot figure out the proper tone and content to coax her son out of hiding.
However, it is apparently intelligent enough to know that it can't do this... and instead attempts to force her to do it.
The funny thing is, given what we learn in the movie, I don't believe she has any better idea how to do it than the machine.
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All answers thus far are in-universe, however I think there is another not-insignificant answer:
The scene suited the portrayal of the T-1000 as a sadist, despite his nature making him presumably incapable of such a thing.
He's been menacing people for the whole of the movie prior to this scene, and capping it off by forcing Sarah to help him endorses his cinema baddie credentials.
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I always assumed it was because the T-1000 was not yet able to emulate Sarah because it had not yet sampled her "by physical contact".
This is backed up by the fact that, after having made contact with her, it then does emulate her a few scenes later.
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In the movie  Terminator 2 , in the scene in the steel mill, the T-1000 machine tortures Sarah Connor to make her call her son John.Watching the movie we learn that T-1000 can reproduce human voices of the people that he touches, so why does he torture Sarah and try to force her to call John? Why doesn't he call John directly?
Why does T-1000 try to force Sarah Connor to call her son?
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<plot-explanation><terminator-series><terminator-2-judgment-day>
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Several reasons explain why T-1000 tries to force Sarah to call John instead of calling itself. First, it may not yet be capable of imitating Sarah. Second, there are signs that it is starting to malfunction after being frozen in liquid nitrogen. Finally, Sarah and her son may well have had a secret code to identify each other, so that h could be sure it was actually his mother speaking. It's reasonable to suspect that John and Sarah have already set up a pre-determined code that enables him to identify when she has been replicated.
T-1000's mimetic powers are weakened after being frozen, melted, and refrozen. Losing effectiveness is shown in some scenes in the film but can be accessed on all DVD versions as a deleted footage from the movie on IMDB. After being completely frozen and then shattered, the T-1000 reveals itself to be defective. its legs are getting as high as the calf, as well as a yellow-striped railing automatically mimic it. The machine does not understand it, but it cannot prevent it. It's reasonable to suspect that John and Sarah have already set up a pre-determined code that enables him to identify when she has been replicated.