The reason the Lions have not yet confirmed the signing of undrafted rookie free agent Randy Phillips is because he hasn't been signed. He is participating in the minicamp going on right now, but he is doing it as a tryout rather than an official member of the team, as the Free Press reports.
Safety Randy Phillips, a rookie from Miami who had been in rookie minicamp with the Lions last month, joined the team on a three-day tryout. The team claimed linebacker Lee Campbell off waivers from the Giants and released long snapper Sam Scroggins, an undrafted rookie free agent from West Virginia State.
There are two possible explanations for why the Lions haven't just signed Phillips. The first and most obvious one is that the Lions simply want to make sure Phillips can still play before giving him an actual deal. After all, Phillips has been injured, so it makes sense that the Lions would want to get a look at him on the field before offering a contract.
The other possible explanation is that the Lions don't want to release someone to make room on the roster for Phillips, so they are having him participate as a tryout. With nothing going on for the month following the minicamp, the Lions may figure that letting Phillips participate as a tryout will give them a chance to keep everybody else around to evaluate. When training camp gets here and the draft picks actually need to be signed, for example, players will have to be released. Until then, though, I wouldn't put it past the Lions to do something like this to keep as many players around as possible for right now.