FYI, AT+CHUP works as mentionned in [Hilo] What is the different between ATH[type] and AT+CHUP? for Hilo.
It only doesn’t work for HL but it is even worse than what you wrote:
With HL6528RD, I tried to hang up the voice call from the receiver phone and it also disconnect the ppp. Have you tried on your side?
It sounds like a bug in HL. I’m using HL6528 SIERRA HL6N,008.10