Clintec, on the TVA824 go into 414 415 416,
day/night/lunch and set all CO lines DIL to to
the 1st voice mail port.

As for your SLT extensions, you should be able to dial 9 and grab a CO.
Make sure in 400 only active CO lines are enabled.
Also check the out dialing permissions for the SLT extensions in 405 406 407, day/night/ lunch
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