Denna will ring the bells on the back door if she really needs out.
She hasn't really "needed" to go out for a long time, either... she gets ample opportunities when I stop by the kitchen for another cup of coffee - if she follows me and heads for the door, I let her out. Normally she goes out first thing in the morning, again around 11am, 3pm, 6pm and 11pm (before bed)... and various random other times, 'just because'.
If Denna needs out during the night, she'll paw at/hit the baby gate on the bedroom door (even though we've been leaving the gate open lately) it makes a metallic ring that wakes me up... if she's insistent enough, I'll get up. Recently, I've ignored the first 'hit' and she's gone back to sleep.