Only one pair is required for a single-line telephone installation, and at least here in the US, most cheap 4P4C cables thus omit the second pair - especially now that landlines in general are so much a thing of the past. I don't know whether the UHK will work with a single-pair cable - I suppose it might, if it uses SPI or I2C or 1-Wire for signaling between the halves, but I'd definitely check the manual before just slapping any old phone cord in and expecting it to work.