SIP response to audio for users

We changed several customers from older Asterisk systems (version 1.8). All of the systems are version 13. Right from the start we got complaints about dropped calls. I am now wondering if the “Audio” the user hears is the difference. I think that now the system will drop a call that is rejected by the SIP trunk provider. Can I change what the system provides to the phones? In a nutshell if it isn’t ringing to be answered can’t I set it so that they get a busy? I don’t want any of the verbal “Can’t be completed as dialed”, etc…