How do you deactivate quiet mode

press # for atleast 2 seconds

Check out this link below to download a PDF manual, software, and more for your phone. Just copy and paste into your browser.

http://www.samsung.com/us/support/detail/supportPrdDetail.do?menu=SP01&prd_ia_cd=01010511&prd_mdl_cd=&prd_mdl_name=SGH-D500

Sound settings (Menu 9.4) from page 77 in your manual:
Use this menu to customise various sound settings.
Incoming call (Menu 9.4.1)
Use this menu to select a volume level and alert type for incoming calls.
• Ring tone: select a ring tone for the call ringer.
• Volume: select a ringer volume.
• Alert type: specify how you are to be informed of incoming calls.
Melody: the phone rings using the selected ring tone.
Vibration: the phone vibrates but not rings.
Vibration then melody: the phone vibrates three times and then starts ringing.

Hope this helps, let me know if I can help you with anything else. It would be very kind of you if this does work for you to please rate me or leave a short note. I would really appreciate it. Thank you for your time.
JimmyC