Consider a call originating in a ‘traditional’ PABX,across a private network (VoIP) to an extension which is on ‘call-forward’ to a mobile phone.
The call will originate in G711(originating PABX), convert to a VoIP standard(private network), back to G711(terminal PABX), possibly back to analogue(POTS line on terminal PABX), probably back into g711 again(Carriers digital exchange) before being forwarded to the mobile network(which encoding? which mobile—anyones guess!!)
This is a simple scenario which involves up to 6 encoding changes.
Yes, this type of modelling is long overdue.