It is highly recommended that your router has and supports Quality of Service (QoS) feature/functions. By enabling the available QoS feature/functions, your router can prioritize the voice traffic over other data traffic; a minimum guaranteed bandwidth can be assigned to this prioritized traffic. And if needed, some consistency with your Internet speed/performance can be somewhat controlled if your ISP cannot guarantee or ensure a minimum level of service.
To view the list of Recommended Routers
and Router Configuration Guides
, go to RingCentral Office@Hand from AT&T Router Configuration Guide
NOTE: Check if your current modem acts as a gateway. You may have to set it to bridge mode before introducing the QoS router to the network.