Phoenix have brought Helm back as Fisher's replacement. He was expensive for them last year, conceding more than two runs per ball and taking five wickets in seven appearances, but was considered unfortunate to miss out on selection at April's draft.
"Whilst we're naturally disappointed to lose Matt Fisher, we're very pleased to have re-signed a talented pace bowler who we know well in Tom Helm," Craig Flindall, Phoenix's general manager, said.
Helm impressed in the early rounds of the County Championship season for Middlesex this year, bowling with good pace, but took 1 for 56 in four overs in his comeback from a minor injury in the T20 Blast on Thursday night.
Atkinson, meanwhile, was an unused member of Southern Brave's squad last year, signing halfway through the tournament as a replacement. He has not featured for Surrey in the Blast this year due to their seam depth but is highly rated by their management and is likely to play when they lose Reece Topley and Sam Curran to England's ODI tour to the Netherlands.