DGS-12-10-28p Slowing down

Since everything is going through the same connection, it's most like that enough bandwidth of that connection is getting used to cause latency, which is impacting the phones. You could conceivably try QoS, but that won't get honored once it leaves your network. Take a look in QoS > 802.1p/DSCP/ToS, and you may find an approach that could help. However, I think that at the end of the day, VLANs are going to be the most helpful approach for you.

Another possibility is the switch getting overworked or needing a firmware upgrade. I'd try upgrading the firmware then working from there.

Thanks masnrock, everything works fine for a week then it starts getting jitter and after a restart it's all good again. I don't think its bandwidth issues.

Does it only happens to phone with PCs behind them, or if it happens with phones as a whole? And if you simply reboot the phone(s), do things work any better?

Depending on which hardware revision you have, the latest firmware came out earlier this year. Take a look on D-Link's site. I'd say start there. But you also may want to check whether you have a lot of devices connecting to the network over the course of the week (which if there is wireless is always prone to happen)