what is call waiting? How to realize it?

Hi, bro. Wish you can consider my solution.
Call waiting means that if a user activates the call waiting service and a third party makes a call to the user when the user is on a call, the user hears a prompt tone of the new call.
By default, users have the service permission. To configure the service permission, perform the following operations:
1.Use an administrator account to log in to the web management system of a U1900 series unified gateway.
2.On the User > SIP User or User > POTS User page, select the user number to be assigned the service permission and click Service Configuration.
3.Under Simple service, select the service to activate and click OK.
4.In the upper right corner of the web page, click save button to save the configuration.

that's mean when a user try to call you when you already have a call
you will get a notification that some one trying to reach you.

Hello, bro! Hoping you are fine. The call waiting means that when you are on a call, another one call you at the same time, you will receive the call, the another one also can hear a prompt tone, also you can transfer to talk with third one. You can configure this feature on the u1900 web page.

Hello.

Thank you for posting on our Huawei Forum.

If a user activates the call waiting service and a third party makes a call to the user when the user is on a call, the user hears a prompt tone of the new call.



Feature Configuration
By default, users have the service permission. To configure the service permission, perform the following operations:

Use an administrator account to log in to the web management system of a U1900 series unified gateway.
On the User > SIP User or User > POTS User page, select the user number to be assigned the service permission and click Service Configuration.
Under Simple service, select the service to activate and click OK.
In the upper right corner of the web page, click click to enlarge to save the configuration.

Feature Usage
POTS Phone
Activating or deactivating the service

A user dials *58#. An announcement is played, indicating that the service is activated successfully.
A user dials #58#. An announcement is played, indicating that the service is deactivated successfully.
Using the service

User A is talking with user B on the phone. User C makes a call to user A. User A hears the prompt tone, and user C hears the ringback tone. Within 15 seconds, user A can make one of the following choices:

Reject user C's call.
User A does not perform any operations, the call waiting prompt tone automatically stops and user C hears the busy tone 15 seconds later. The call between users A and B is not affected.

End the call with user B and talk with user C.
User A presses the hookflash button and then 1 as prompted.

Hold the call with user B and talk with user C.
User A presses the hookflash button and then 2 as prompted.

When talking with user C, user A can press the hookflash button and then 2 to resume the call with user B. User C hears the call waiting music.

If user A hangs up when talking with user C, users B and C hear the busy tone. If user C hangs up when talking with user A, user A can press the hookflash button as prompted to resume the call with user B.

IP Phone
Activating or deactivating the service

On the LCD screen, choose Apps > Pref > Call Setting > Call Waiting.
Select Enable or Disable to activate or deactivate the call waiting service.
Using the service

User A is talking with user B on the phone. User C makes a call to user A. User A hears the prompt tone, and user C hears the ringback tone. Within 15 seconds, user A can make one of the choices shown in Figure 1.

bro,
Hoping you are fine.The 5# Reply to your question in detail, I hope his answer is helpful to you, if you have any other questions, you can also consult us by posting.Thanks.