Call forward busy tone

Comments are Incorporated and the Test Cases are Successful.

1-Test Case Suggested as under is Passing :

• Establish a regular call between line 1 on Norrland and the reference device
• Call the special call like 21 from line 2 on Norrland
• After the call on line 2 is established and before it ends, hang up the handset on the reference device
• Make sure the busy tone is played on the handset connected to line 1 on Norrland Similar test cases executed.

2 -play_busy_tone is moved as a property of subchannel and its initialized as True by default, and reset as other flags in the subchannel.

3- In Function ast_lantiq_indicate, we are checking for cases where we can stop the busy tone line is #21#, and Enable busy tone example in a normal call A to B.