Here is a claustrophobic thriller that unfolds entirely inside a small telephone exchange on the island of Alderney in the early 1960s; it features just one actor visible throughout, with other characters only heard through the telephone lines. Writer-director-producer Oliver Pearn, making his feature debut here after working as an assistant director and the like, is laying down a crisp white calling card by showing off how economically he can tell a story. While this sometimes feels less like cinema and more like a stage piece or even a radio play, the end result is reasonably engaging and propulsive. Pearn gets a crucial assist from lead Victoria Lucie who plays Agnes, an operator who spends her days inside a hut working the complex machinery of the exchange; these were the days when you couldn’t just dial a number to call someone but had to be literally connected via wires being plugged into sockets. With a few calculatedly planted lines we work out that Agnes is a personable young woman, the proud winner of a Best Voice competition for phone operators. However, she’s keen to be taken a bit more seriously, not just by her customers on the island but also her fiance Frank (Thomas Bliss), with whom she checks in regularly throughout the day to discuss wedding plans and share chit chat. A confused elderly woman called Shirley (the great Harriet Walter no less) calls in from a nursing home sounding frightened; that is followed up by a call from Martha (Joanne Rogers), a nurse who hints that she’s been kidnapped. Agnes tries to raise the alarm and help the women but former co-worker Betty (Sally Geake) is now at the cop shop and is a bit patronising towards Agnes, who failed to pass the police exam herself, and doesn’t seem to be taking the situation seriously. With help from Frank on the ground, Agnes pieces together what is going on, and where Martha and her supposed kidnapper Harold (Royce Pierreson) are – but has she misunderstood something? This isn’t quite as gripping and bravura as other one-isolated-person-interacts-with-the-world-via-technology films such as Peter Strickland’s eerie Berberian Sound Studio or Steven Knight’s niftily plotted Locke, but it holds its own. Lucie nails Agnes’s voice: a very specific blend of regional English accent diphthongs just muffled by the plummy vowels and repressed alveolar approximants to ape the received pronunciation folks expected telephone operators to use back in the day. As with the costume and set, real consideration has gone into the period details here. On the Line is released on 22 January on digital platforms.