Another factor may be people hanging up on pollsters or screening calls and not picking up calls from numbers they don’t know.
Consider the government’s abysmal failure running the do not call registery. My land line and cell phone are inundated with solicitation calls every day. As a result I will not pick up a call from a number I do not know. Legitimate callers will leave a message, robocalls, pollsters, and solicitors move on when voice mail connects.
I don’t answer calls from numbers that I don’t recognize. I have a specific ringtone on my smart phone numbers that I don’t recognize, but I always do a search for the number and if it is a important number I call it back, most though they leave a voice message. Otherwise it goes in my reject folder. My house phone has a voice ID and announces the number that calls. This way I do not even have to answer the call, but as with calls to my smart phone I do a search for the numbers