altistart 48

It's been a while since I messed with one of those pieces of junk, but I seem to recall that the PHF display means Phase Loss fault. The starter has detected that at least one of the phases is not pulling enough current, which is what would happen if you lost a line fuse. But because the condition can be satisfied if ALL 3 phases are too low, you can get a false trip if the motor is not pulling enough current after a pre-determined (by the factory) amount of time, i.e. 30% of the unit rating by 3 seconds is typical. This happens a lot with pumps unfortunately, because the load does not couple with the motor until the water flows, and it may not flow during the earliest parts of a long ramp.

There are several solutions:

1. Decrease the ramp time. This happens sometimes because people seem to think a soft starter is like a VFD and they can ramp it for a long time. Not so, especially on centrifugal pumps. Centrifugal pumps must typically be accelerated to 80% speed in no more than 5 seconds total, sometimes even less if they are fluidic bearings.
2. Disable the Phase Loss protection. In some cases the low acceleration rate cannot be avoided and in order to prevent this false tripping, you must disable the Phase Loss all together. I'm not sure if the ATS48 has it, but on some I would then enable the Phase Imbalance protection in lieu of Phase Loss, it will do the same thing but without the risk of false tripping.
3. Make sure your soft starter is sized correctly. Some people think that "bigger is better" and buy a soft starter that is much much bigger than necessary for the task. Unfortunately because the Phase Loss trip threshold is based on the UNIT rating, not the motor programmed FLA, you end up with this situation. If so, it will not work (unless maybe you do 1 or 2 above) and must be replaced with a unit closer to the motor size.