I've had good experiences with calling, but I upvoted anyway because I do believe there are hotels that are stuck in their ways and have no flexibility to offer a better rate to someone who is trying to price-match booking.com.
I've been using HotelTonight a lot as it lets me pick a date I might want to go somewhere and it sends me updates on decent hotels as the price goes down. But I use TripAdvisor to check first if it's a good spot to stay.