Only five players who have worn a Phillies uniform for the majority of their careers have had their numbers retired by the organization: Mike Schmidt, Steve Carlton, Richie Ashburn, Robin Roberts, and Jim Bunning. All five of those players are also Hall of Famers. This is not a coincidence. The Phillies have long had an unofficial rule that only Hall of Fame players are eligible to have their numbers retired by the organization.

Source: Is Curt Schilling a Hall of Famer? Would he go into Cooperstown as a Phillie? | PhillyVoice