Phone is constantly busy

HI Michael,
I have this site for you on how to set dialing tone on your IP Phone. Hope it helps.
https://www.google.com.ph/?gws_rd=cr&ei=8CNaU5KaFYKiyASIhoKACA#q=aastra+6753i+ip+phone+user+guide ( please choose the 1st link)

All incoming calls get a busy signal

Sent yourself a message from another phone then delete it with 3 3 7. That should do it

here is

If your phone company provides the service, you should have a code to enter. If YOUR telephone switch is doing the forwarding, you will also have a code to enter.

Basically, you enter the code, then dial the number and hang up. To unforward, you dial a similar code, and hang up the phone.

if the phone is an AAstra phone model 9116 (look on the underneith) then hit Options then down arrow (under the viewer) to the indicator message then hit delete and then save
if it is a 9216 or 9316 model then slide the little bar(under the view glass to the right and hit the button under the slide and down arrow to option 4 and hit the 4 button and turn of the indicator. it will work properly imediately and resume as it should..
to prevent this , after you listen to your messages hit the * button twice before you exit your voicemail system

http://www.metacafe.com/watch/515782/how_to_dial_an_emergency_number_without_pressing_the_keypad_of_y/

if it is part of a set of phones it could mean that another one in the set is 'on' or 'off the hook' or not hug up propperly.

If that model phone has a rj14 jack ( standard phone jack) and when you plug it into a known good wall jack and you can draw dial tone then it will work. If not it requires a KSU to interface with the lines and that's a lot more equipment than you need for 1 or 2 lines. Thank you. Jer