Good advice about the 'keeping in' routine.
If you don;t already, ensure that you feed them at the same time every day. Make a loud tapping sound on their food bowl, or call them, which ever suits, as you put the food down.
When you do let them out, make it about twenty minutes before feeding time, and then do your tapping / calling routine from the back door - they'll hear it, and come running.
A couple of days like this, and you should be able to drop the 'noise' bit, they will come anyway.