Ports either work or they dont.
If you have any audio then your port is already correctly open to your machine.
There is no way that opening a port will affest call quality.
These are the things that will:
Dell Optiplex (there is a fix)
Bad local network
Bad internet connection - not enough bandwidth - excessive jitter
VOIP provider trouble
You can seart troubleshooting by running some basic tests like found here:http://www.pingtest.net/