The PHONEFEATURE_ constants list the operations that can be invoked on a phone using this API. Each of the PHONEFEATURE_ values correspond to a TAPI function with an identical or similar name.
PHONEFEATURE_GETBUTTONINFO (0x00000001)
PHONEFEATURE_GETDATA (0x00000002)
PHONEFEATURE_GETDISPLAY (0x00000004)
PHONEFEATURE_GETGAINHANDSET (0x00000008)
PHONEFEATURE_GETGAINSPEAKER (0x00000010)
PHONEFEATURE_GETGAINHEADSET (0x00000020)
PHONEFEATURE_GETHOOKSWITCHHANDSET (0x00000040)
PHONEFEATURE_GETHOOKSWITCHSPEAKER (0x00000080)
PHONEFEATURE_GETHOOKSWITCHHEADSET (0x00000100)
PHONEFEATURE_GETLAMP (0x00000200)
PHONEFEATURE_GETRING (0x00000400)
PHONEFEATURE_GETVOLUMEHANDSET (0x00000800)
PHONEFEATURE_GETVOLUMESPEAKER (0x00001000)
PHONEFEATURE_GETVOLUMEHEADSET (0x00002000)
PHONEFEATURE_SETBUTTONINFO (0x00004000)
PHONEFEATURE_SETDATA (0x00008000)
PHONEFEATURE_SETDISPLAY (0x00010000)
PHONEFEATURE_SETGAINHANDSET (0x00020000)
PHONEFEATURE_SETGAINSPEAKER (0x00040000)
PHONEFEATURE_SETGAINHEADSET (0x00080000)
PHONEFEATURE_SETHOOKSWITCHHANDSET (0x00100000)
PHONEFEATURE_SETHOOKSWITCHSPEAKER (0x00200000)
PHONEFEATURE_SETHOOKSWITCHHEADSET (0x00400000)
PHONEFEATURE_SETLAMP (0x00800000)
PHONEFEATURE_SETRING (0x01000000)
PHONEFEATURE_SETVOLUMEHANDSET (0x02000000)
PHONEFEATURE_SETVOLUMESPEAKER (0x04000000)
PHONEFEATURE_SETVOLUMEHEADSET (0x08000000)