If you are unable to hear properly it shows problems in download side, I’ve had 3Mbps connection and it worked great though sometimes voice was garbled (either incoming, outgoing or both), now it is 5Mbps it is still same (occasional garbling on either side). People with DSL connections (around 1Mb down, 256kbps upload) have used VOIP with no issues at all.
You can check your connection speed at:
Speakeasy - Speed Test