I'm a huge fan of Ubiquiti products, https://www.ubnt.com/, have used them in several non-profit and educational installs as well as my own home. Here are some thoughts out loud:
Voice is one of the toughest forms of traffic to transport across networks, it's time sensitive, can't really be buffered, and there is no recovery from dropped data. That being said, even if you're VPN'ed into your company, there's still a chance that latency or dropped packets can be happening at any point along the path of the connection - that very well could be outside of your connection and control.
Your router is important, but most modern name brand routers can keep up with the traffic flow, basic NATing/firewall, etc. The router is mainly enabling you to get online and share your ISP's connection throughout your home. The Wifi component of that is just allowing you to share it out over a different medium.
Usually the issue is Wi-Fi related, whether your neighbors are crowding the frequency spectrum or the Tx and RX strength is just not strong enough.
Wireless to Wireless relays can be latency monsters. The latency from the radio hops can be significant and not an ideal situation for VOIP.
Some suggestions:
Disable WiFi on cable provider router, wire in an AP or two.
Keep WiFi enabled on cable provider router, use an Ethernet over Powerline adapter to the distant device. Keep in mind you don't need a lot of throughput for a VOIP phone, more a solid connection that doesn't drop or suffer from latency spikes.