I'm not sure what the opposing pool is, but my guess is Device Rating X 2. @Herr, @Tec, you two care to weigh in?
This is actually covered by the core rules for once:
Voice recognition systems require a vocal response from an approved user’s voice, usually within a certain amount of time. If the response is not given within the time limit, or someone not approved answers, the system sounds an alarm. Characters can only defeat voice recognition systems by “speaking” with the voice of an approved user—by using a recording, some other simulation, or the real voice. Voice modulator cyberware (see p. 452) can also be used. An Opposed Test must be made between the voice recognition system and the equipment used to bypass it; whichever generates more hits, wins.
Unusually, this is generally just an opposed test between the Device Rating, not Device Rating x 2. As per Print Scanners:
"If a fake print is used,
make an Opposed Test between the duplicate and the maglock rating; if the fake wins, the maglock accepts it."
And facial recognition:
"Prosthetic makeup and biosculpting can be used with varying degrees of effectiveness against facial recognition;
make an Opposed Test pitting Disguise + Intuition [Mental] against the Device rating."
Device Rating x 2 is mostly used for SIN scanners, but rarely for other security devices. However, I think this is mostly because when "equipment" is referenced it's usually in the form of Maglock Sequencers or Voice Modulators and the like, all of which have similar 1 to 6 ratings. Using the default RAW of Device Rating as the opposed pool does make it relatively easy for a social engineer of Chaim's talents to fool a voice print scanner, as even a Rating 6 device is highly unlikely to beat his 16+ dice pool (of course, a GM could easily rule that tailored pheromones do not apply

).
I'd be tempted to use RAW in this case, but that's mostly because I usually play in low-powered campaigns. I'd also be OK with device rating x 2 as the opposed pool since Chaim would be using his real voice and thus his skills, and not a recording where that device rating would be pitched against the maglock rating, but I'll leave the final decision up to the GM.