Don’t worry, I won’t judge you for having a land line. Some people do. Some people just can’t give up the feeling of security. I totally understand. But if your cordless phone isn’t reaching all the way through the house, a cell booster isn’t going to help you.
Cordless phones use the consumer bands of 900, 2400, and 5800MHz, which are designed for very short range communications. They’re still licensed by the FCC, but not regulated as heavily because they’re designed for very low power devices like your cordless phone. Cell boosters boost signal in the 700, 800, 1900 and 2100MHz bands and therefore don’t work for cordless phones. It’s designed that way.
If your cordless phone doesn’t reach everywhere, consider changing the battery or buying a newer 5.8GHz model which has better range. Because demand for land line phones is down, prices for them have dropped and it certainly shouldn’t be a problem to find the one you want.