How to send a busy signal when a subscriber is talking in asterisk?

Question

Asterisk

Asterisk Settings
By default, each user has the ability to use multiple connections at the same time. What is not good for sip-devices with one line. In the config, you can force call-limit=1
When setting a limit on one connection from one account in the config, or if the device supports only one line - when you call such a device, at the moment when they are talking on it, the answer is: "Currently moment no one can accept your challenge." Also, this response is heard when the sip client is not registered (the device is turned off). And we are misleading the caller.
How to make a call to a subscriber who is talking to receive a "busy" signal, or to announce "The subscriber is currently talking" by voice?

Answer 1

exten => _X!, 1, GotoIf($[${SIPPEER(${EXTEN},curcalls)}<${SIPPEER(${EXTEN},limit)}]?num_free)
same => n, Answer()
same => n, Playback(abonent_is_busy, noanswer)
same => n, Hangup
same => n(num_free), Dial(SIP/${EXTEN},,t)