Thank you so much for the feedback and support.
I have made the changes you suggested. "Please try changing the PDP type to IPv4/IPv6 by navigating to Network → Interfaces → General and editing the mob1s1a1 (or mob1s2a1) interface. To enable DHCPv6, edit the lan zone in the same menu, and under the DHCP settings, in the IPv6 Tab, set the DHCPv6 to server mode."
But I haven't done this: "Network → Firewall → General Settings and disable the Automatic helper assignment" because I need to test the other changes and ensure that the voice calls problem persists before I embark on this.
I will keep you informed on what happens with the changes I just made.
Also attached a new troubleshoot file to the original message.