BT has introduced a new service called BT SmartTalk, which it claims can save you money by offering access to your BT landline tariff for calls placed on your mobile phone.\u00a0The BT SmartTalk service works through a free app to essentially give you a VoIP (voice over IP) network phone. When you place a call through BT SmartTalk, it will be billed at your BT landline rate, because it's using your data connection, rather than your mobile cellular connection.Of course, the first catch is that placing calls over 3G may cost you money if you have no data on your mobile phone tariff, but if you're on a free Wi-Fi network, like at home or work, then the calls you place will be at BT landline rates when using SmartTalk.\u00a0And by BT landline rates, we mean the rates within your BT calling plan. If you have free evening calls, you'll get free evening calls through BT SmartTalk.This arrangement obviously comes with some advantages: numbers that are free on BT landlines, 0800 for example, can now be free when you call them from your mobile phone using SmartTalk. In addition, you won't have to up landline minutes on your mobile tariff if you don't want to.\u00a0Because SmartTalk can use a Wi-Fi connection for this service, it's equally applicable anywhere in the world. If you're in a hotel in the US and want to call home, SmartTalk will let you make that call at your normal BT landline rate, but bear in mind that any free calls (like free evening calls) will apply only if it's evening in the UK.\u00a0Set-up is relatively straightforward. After installing the app (Android or iPhone), you have to supply your home phone number and mobile number and go through an authorisation process. This will let you link your mobile to your BT landline account to activate the service from that device.There are lots of safeguards to ensure that the bill payer knows a SmartTalk device has been activated, so don't worry about people freeloading on your landline.Within the app you can access your device contacts and from our initial play it's easy enough to navigate and use.\u00a0Once you're in a call, you get an indicator to tell you whether you're on Wi-Fi or 3G, as well as normal in-call features such as loud speaker or number pad. Call quality in our initial tests seemed reasonable too, but we've not used it enough to really make an evaluative judgement.One oddity we found, however, was that the app didn't automatically disconnect when a call was ended by the other person. It goes silent, but the timer keeps rolling: you have to end the call manually.\u00a0\u00a0You can link up to five devices, so if you have a family of smartphone users in the house, each person can get connected. This comes with another advantage: if one person is on the phone - the landline - then others can still call out via Wi-Fi and it's all billed to the BT account, rather than the individual mobile accounts, so you can really use those free BT calls.But there are some things to be considered here. Using BT SmartTalk wholesale could end up costing you more, because you might be transferring paid-for landline calls to your BT bill and not using your free network minutes in your mobile phone contract, even worse, you could be doing it roaming on 3G.So our advice is to check what you get free on your BT tariff and what's free on your mobile and consider which avenue is best for your when you go to place a call.There is a safeguard for this in the settings however. You can opt to select what types of calls are automatically routed through BT SmartTalk. Freephone numbers, 0845 and 0870 or UK landlines can be selected at will. When you then place a call through the regular dialler app (on Android at least) it will switch to BT SmartTalk when\u00a0appropriate, which is quite clever.To decide whether BT SmartTalk is going to offer you an advantage is really a case of determining whether your BT tariff offers you better value for money over your mobile tariff or other third-party services like a Skype subscription.Ideally, you can better use the free BT calls you're already paying for, thereby saving yourself some cash.BT SmartTalk is available to download now from iTunes or Google Play\u00a0now.