Phone plans change all the time. If you've had the same phone plan for a while, whether it is a home or mobile phone, you might find you're on a deal that doesn't suit your spending habits.
Look around at what other carriers can offer. Some do combined deals for your home, mobile and Internet. Some have call caps that are great if you're a heavy user. If you're a light user, you might want to look into naked dsl and do away with your home phone entirely.
Just be careful that you take into account any fees to switch or setup. If you are switching for a very small saving, you might find that the cost of the switch and setup fees are higher than anything you'd save in a year. The same warning applies to constantly switching between carriers or ISPs - always allow for the reconnection fees in your calculations.