Both players count toward the 90-man roster for Indianapolis and can be activated at any time.
Fountain, 23, was drafted by the Colts in the fifth round out of Northern Iowa in 2018. He signed a four-year deal worth $2.7 million but was released by the team during final roster cuts.
The Colts signed Fountain to their practice squad and promoted him to the active roster later in the season. He appeared in one game in 2018 and did not record a stat.
Phillips, 26, originally signed on with the Jaguars as an undrafted free agent out of Illinois back in 2017. He spent over a year in Jacksonville before he was waived in 2018.
The Colts later signed Phillips to their practice squad before promoting him to their active roster a few weeks later.
In 2018, Phillips appeared in six games for the Colts and recorded five tackles and no sacks.