Whether you're using VoFR, VoATM, or VoIP, if devices in the network are not properly configured, you can experience problems with voice quality. Also, if there are large delays or the available bandwidth is overwhelmed, there will be problems with voice quality. With VoIP devices, you can typically configure IP level QoS (e.g., IP Precedence, IP Tos, or DiffServ). This can help to prioritize delay sensitive traffic such as voice and video over data traffic.
You really have to look at your bandwidth utilization and understand the maximum amount of bandwidth you wish to use for voice traffic. For a VoIP call using compressed voice (G.729a), the approximate bandwidth used (one way) is 24 kb/s (including Ethernet and IP overhead). For a VoIP call using uncompressed voice (G.711), the approximate bandwidth used (one way) is 87 kb/s (including Ethernet and IP overhead). This will help you to understand what type of broadband service plan you need.