Possible fixes to try:

Update the firmware. You may need to experiment with different firmware versions.
Try disabling SIP Helper if that is being used.
Try UDP timeouts of at least 360 seconds especially if the phone isn't ringing.
Try setting the firewall to the low setting.
Try excluding content filtering for the IP range of the phones.
Try disabling Attack Checks if that is being used.
You may need to create an outbound access rule for the phones.
Try disabling DoS protection if that is being used.
Try disabling port scanning if that is being used.
Try disabling SIP Transformation if that is being used.

It is important that you use "Consistent NAT" where the router keeps the same NAT port for the phones for as long as you are connected. This isn't an issue if you're not using Network Address Translation.

The NAT Entry Expiration needs to be at least 6 minutes or 3,600 seconds because Polycom phones send a REGISTER refresh message every 5 minutes. This is not an issue if you're not using Network Address Translation.

The following functions need to be disabled or not used. They can cause call quality issues and other intermittent problems. The spreadsheet covers which devices have these settings and how to disable some of them.

SIP ALG
Stateful Packet Inspection (SPI)
Deep Packet Inspection (DPI)
Application Layer Access Control
Intrusion Detection/Intrusion Prevention System (IDS/IPS)
- IDS/IPS may limit packet streams causing intermittent problems during high call volumes.
WAN Acceleration
- WAN Accelerators use header compression which can cause jitter.
Port Filtering such as UDP Flood Protection
- Flood protection can limit bandwidth which causes call quality issues during higher call volumes.
Load Balancing
- This can cause jitter and increased packet loss.
Auto QoS when used in combination with Polycom phones
- This can cause sound distortion or incorrect volume levels.
Dynamic ARP Inspection
Green Ethernet
- This can cause intermittent signaling and media traffic problems.

Related Pages:

Modem, Router, Gateway & Firewall Compatibility Guide

Free Line Test (Packet Loss, Latency & Jitter)

Free Speed Test (Upload, Download & Latency/Ping)