No polls CAN be reliable nowadays. No one answers their home phone unless they are a dinosaur due to spam calls. Many don’t answer their mobile unless they know the caller. Most don’t want to spend time answering a poll. Small sample size.
In addition to the factors you mentioned, I suspect that there are a number of Trump supporters out there who are reluctant to tell a stranger on the phone that they support Trump. People don’t want to be perceived as being racist sexist homophobic and deplorable.