Hi, friend!
The possible causes are as follows.
Either the number does not exist in the office, or the prefix of the calling number from the attendant is not configured on the voice server.
You can run the show prefix dn * command on the voice server to view the prefix information.
For example, if the number you call is 6008, run the show prefix dn 6 command. If the prefix does not exist, run the following command to add the prefix:
config add prefix dn 6 callcategory basic
callattribute inter cldpredeal no minlen 0 maxlen 24
I hope I could help you!