Generic answer: if you want to retain the source IP address of a client, then you must avoid any network address translation (NAT) in the network. With what you describe, you're probably doing NAT on the Debian OpenVPN server.
Please note that if you stop doing NAT on that server, your clients may be able to reach your VOIP Server using their original IP addresses, but you have to make sure that the VOIP Server can also reach the clients. So you'll need to make sure that all relevant hops know that to reach the 10.16.19/0/24 subnet, the traffic needs to be routed to the OpenVPN server which in turn needs to route it through the correct tunnel.