Landlines suck. The phones themselves are a throwback to the days when phones were for, y'know, phoning people, and you can't even play Snake on them! There's one thing going for them though: they're cheap. This app lets you access landline prices when you're out and about, or better, abroad.
BT's SmartTalk (if you're not a BT customer, stop reading now/start turning lime green with envy) lets you use your landline calling package through your mobile phone. You install the iOS or Android app, register your BT account (up to five users per account), and Bob's your incredibly chatty uncle. The app works over 3G or WiFi, despite what BT's app description might erroneously say.
Once upon a time, in a land not too far away (well, just outside the UK), I racked up one of those insane mobile bills, the type you hear whispered about but can't really believe. It was all from calling. The greatest use of this app is to make voice calls (probably over Wi-Fi) from overseas for non-ruinous amounts of money. It could also be helpful for calling certain 08something numbers, which cost way more to call from a mobile for some reason.