[7] De Caestecker spoke of having "felt a degree of responsibility when it came to portraying the career [of a call handler] accurately on screen", commenting that he experienced what it was like in a live control room, and that "they do such an amazing job [and] potentially don't get the credit they deserve", meaning it "was one thing we were very conscious of, just making sure we did that respectfully".
Emily Watkins of i only reviewed the first episode, yet said it "put hardly a foot wrong", that "[p]erformances from both De Caestecker and Vanderham were exquisite", and "[w]ith its fresh-feeling premise, fleshed out characters, and just enough twists to deliver escapism without compromising believability, [it] has all the ingredients to emerge as one of the year’s standout dramas", giving it four stars.
[13] Lucy Mangan of The Guardian awarded the same, praising the cast and setup, calling it "a thriller that takes a tremendous, hooky premise, then builds around it with loving detail", and that the "convolutions of the ... narrative seem to arise organically and never strain your credulity.
"[14] James Hibbs from the Radio Times also awarded it four stars, praising the cast - De Caestecker in particular - as well as "the visuals", all of which "makes its [sic] an atmospheric and compelling watch, with any other qualms tempered by its sheer entertainment value", yet noted "the show's delight at concealing its multiple twists and turns [is] a tactic which works wonders at the start of the series but occasionally reaches the point of frustration midway through", exacerbated by "hefty dose of childhood flashbacks, a narrative device which always needs to serve a purpose, yet here largely chooses atmosphere over illumination, repeatedly slowing down the otherwise propulsive action".
[16] Dan Einav's two-star review in the Financial Times remarked that the "trauma-filled flashbacks [were] stand[ing] in for careful character development", while deeming the show "sadly prosaic", "dry and uninspired", with the "scripting ... overwrought and the plot contrived".
[17] Sean O'Grady of The Independent called the flashbacks "annoyingly extensive, intrusive and confusing stylised" - "no better symbol of the creative impoverishment of television drama departments than this" - and that the show "is let down by too many" of them and "disjointed storytelling" that requires suspension of disbelief, somewhat "redeem[ed]" by "the performances of the leads" - especially Vanderham, her characterisation "disturbing but compelling" - in a three-star review.