The best way of deploying QoS is using bandwidth guarantees/rate limiting. Depending on the capabilities of the device, you may be able to define the Press8 IPs as having a guarantee (our subnet is 188.8.131.52/22), or you might need to try defining non-voice traffic (such as port 80, 25, etc), and specifying a rate-limit there. Using VLANs of course can also help you define these classes for applying bandwidth rules to. The Polycom phones support 802.1q tagging. Last, if available bandwidth is slim at the customer location, we can use low-bandwidth audio codecs on the phones, specifically G.729 resulting in approx 32kbps up/down per call as opposed to ~95kbps for G711/G722.
One thing to always check on the router is whether or not it is SIP-aware, as they generally cause more harm than good, and we pretty well always look for this and disable it where applicable. This is usually referenced as a SIP ALG (application layer gateway) on the router. It can also be helpful to disable deep packet inspection filters, may also be labeled as SPI / Stateful Packet Inspection. This is done to prevent the routers from incorrectly identifying/filtering legitimate SIP traffic.