it all depends on the network at the same time the traffic coming in or out in the network that affects the quality call when receiving or placing a call on a VoIP networks, the best way to explain to the client is give him/her the lowdowns on the technology how it works without being too technical on him/her..use an analogy in explaining it,..probably like this approach by explaining to him/her when you make a phone call over an VoIP networks, tell the when they make a phone call their speech is being converted into digital signals and being transmitted over the network and later being reassemble them into one piece(regardless if they calling into analog phone receivers..so called POTS(plain old telephone system), the following factors that may affect the quality of call if theres too much traffic over the network, making the digital signal takes a while to be assembled thats why sometimes you would hear that voice being echoed or choppy and ideally a VoIP works in an ideal world if the networks not too crowded when receiving / placing a phone call..so i hope i give you an idea how to explain it on your client without making you totally an alien to them who speak a different language.