Asterisk: how to notify a client about his transfer to a certain number (attended transfer)?

Question

Asterisk

Hello!
Actually, the question is indicated in the topic.
The task is to make sure that the client receives a sound notification when a call is transferred to a specific number (we are talking about attended transfer). Something like this: the client calls, gets to the operator, he transfers him to a certain number, the client hears the message while he is on hold.
An important point: we must check the number to which we are transferring the call, and depending on this, decide whether this message is needed. For example, we must play the message only when transferring to extension number 101 - then when transferring to another number, the client should not hear anything. Accordingly, putting a notification in music on hold is not an appropriate solution.
Thank you!
UPD: the answer was found in a comment by alegzz:
via ChanSpy - it is necessary to create a call file of the form when calling / answering 101

Channel: Local/[email protected]
Application: Playback
Data: файл_с_анонсом

in the notify context:

Answer();
ChanSpy(<канал клиента>,qw);

Answer 1

set the TRANSFER_CONTEXT variable, in the context if ${EXTEN} 101, make Dial with parameter A(file with announcement), otherwise without. or do you need ready-made extensions?