Previously educated at the Edinburgh College of Art, by 1981 he joined the infamous 80’s sensation ‘The Human League’, where he would play both keyboards as well as lead guitar. He also made contributions as a backing vocalist to the hits such as ‘Don’t You Want Me’ and ‘(Keep Feeling) Fasination’.

Callis co-wrote many tracks and following his departure from the band in 1986, he has returned several times either to play keyboard, or to offer help with songwriting.

In 1985 he teamed up with Feargal Sharkey to write Sharkey's "Loving You" which reached number 25 in the UK Singles Chart.