In the past, if you wanted to take your landline on holiday you'd need to unplug it from the wall and find space in your suitcase. But now with BT SmartTalk there's an app for that, letting you make calls at your landline rate from your mobile when you're out and about, or even out of the country.
SmartTalk is a VoIP app for your iPhone or Android blower that lets you talk on your mobile, but charges you at landline rates. That includes your free evening and weekend calls, if you have them as part of your landline package.
Landline rates apply when you're on holiday, so no hideously expensive international call rates hitting your bill when you get home.
And you can add up to five mobile phones to your account, so the whole family can take advantage of the rates, including when someone is on the landline.
Another advantage is that you can use the app to call 0800 freephone numbers, as well as 0845 and 0870 numbers, instead of being charged loads because you're on your mobile.
Whether SmartTalk is worth it depends on which BT package you choose, and whether those landline rates are better than the mobile network tariffs on offer. You could also choose rival VoIP apps like Skype or Viber.
The app works on Wi-Fi and 3G, although you need a data plan for your phone to make calls over 3G. If you're a BT Broadband customer and you're in a BT Wi-Fi hotspot, you get free Wi-Fi access too.
Would this persuade you to switch to BT? What are your tips for saving money on your mobile? Tell me your thoughts in the comments or on our Facebook page.