Comments:

Robert W Phillips was a soldier from Company F of the 38th United States Volunteer Infantry Regiment.  The 38th Volunteers was created during the Spanish American War and served in the Phillipines.  It was probably during this time that Phillips would have been convicted by court marshal of some military offense and sentenced to the prison on Alcatraz, where he died.  Army inmates on Alcatraz were designated either as "general prisoners" or "military convicts."  General prisoners were sentenced to Alcatraz for a period of time, but after that had been served they would be returned to their units and allowed to serve the rest of their enlistment.   Military convicts like Phillips, however, had been dishonorably discharged and were spending the rest of their enlistment periods (or longer, if the offense was serious enough) in confinement. 

Phillips' tombstone is evidently a post 1947 replacement as the stones for other Alcatraz convicts are devoid of religous emblems and unit names as a reflection of their status.  Phillips is the only member of the 38th Volunteers interred on Angel Island.