You will get a recording stating "licence limit reached please contact your administrator".
This can be located in the PBX's folders - go to sub directory:
C:\ProgramData\3CX\Instance1\Data\Ivr\Prompts\Sets\8210986B-9412-497f-AD77-3A554F4A9BDB
You can change it to an engaged tone or something like that. The file you want is licence_limit.wav