Good article by Robert Poe at VoIP News on why mesh WiFi technology may harm the quality of VoIP phone calls:

Wifi mesh has a number of benefits for deployments covering large areas and large numbers of access points. Connecting APs to a high-speed Internet gateway via wireless rather than wireline/cable links makes it easier to blanket broad areas with coverage. It’s cheaper than running cables to APs on light poles, for example, and can provide substantially higher throughput than connections that rely on 1.5Mbps T1 lines. And because the architecture allows multiple redundant routes, it can be more reliable than landline-based backhaul.

… But the technology has inherent disadvantages when it comes to carrying voice. The more wireless "hops" a call has to traverse, the more likely it is to suffer increased latency, especially when the network gets busy and the spectrum crowded. The result can be voice quality problems involving factors such as cadence, echo and jitter.