Use of TONE_DETECT to detect dial tone in call (Asterisk18)

I am trying to use TONE_DETECT in a call that is made into a call centre
and placed on hold, when the call has been answered by the agent (denoted
by a dial tone of 2 seconds of dual tone 350Hz + 440Hz followed by 4
seconds of silence (North American dial tone), to act as a trigger t to
bridge a call to our agent. I am using

exten => s,n,Set(TONE_DETECT(0,,d(5)crg(tonedetect-dialagent,s,1))=)

which should look for dial tone on the rx channel with a decibel threshold
of 5 (tried default and didn't work), and then jump to the context
tonedetect-dialagent.

The documentation on the use of the options for TONE DETECT offers no
explanation on the syntax to use, so I have guessed the above.

Does anyone know if this is correct usage of the function? Also it is still
not detecting dial tone.

I noticed in Asterisk 20 there is a new function called WaitForTone() App
on its way

https://wiki.asterisk.org/wiki/display/AST/Asterisk+20+Application_WaitForTone

Anyone know anything about this?

Thanks

Steve

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