Hello,
which 3750 do you have ? The 3750X ? The sample config below uses tthe default Vlan for voice traffic:
interface gigabitethernet1/0/1
mls qos trust cos
switchport voice vlan dot1p
If you have a separate voice Vlan, the config would be:
interface gigabitethernet1/0/1
mls qos trust cos
switchport voice vlan x
The 3750X also has the option to detect Cisco IP Phones, using the interface command 'switchport voice detect cisco-phone'.
Check the link below for details:
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swvoip.html
