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.

Common phone cable in Finland seems to be 6P4C, at least the ones I had.