AT&T
AT&T has only recently begun manufacturing the phones it sells.
Hitachi
made the AT&T 1100, 1200, and
1710.
Oki
made the 3030,3035,3430 and 3730,
Mitsubishi the 1300,1300C, and 1400.
The 3610 was manufactured by
NEC.
Who knows who the model 3000 was made by.