You would need an OBi110 to connect to a landline.
You would also need a softphone on your mobile phone.
The easiest is to use OBihai's OBion softphone.
Each OBi device/softphone has a unique number assigned to it.
You would bridge the inbound landline call to the OBi number of your OBion softphone.
It requires changing one line of code in the OBi110's config.
It has been reported that the OBion softphone is not working with IOS8.
Install any softphone on your mobile that supports SIP.
You would have to define a dummy SIP trunk on the OBi110 and bridge the call to the IP address of your mobile.
Edit:
The OBi110's defaults are for a N. American landline.
ianobi would be the one to help you with any changes needed for the BT landline.
You will need to change the Line Port Digitmap to make outgoing calls from your OBi110 to your BT Line. (For non-UK readers BT here means British Telecom, not Bluetooth!)
MarkObihai recommends this for a UK landline:
Physical Interfaces > LINE Port > DigitMap:
(999|112|101|111|116000|116111|116123|1471|1571|0[15]xxxxxxxxx?|0[27]x xxxx xxxx|0800xxx xxxx?|0808xxx xxxx|08001111|08[47]x xxx xxxx|0845464x|03xx xxx xxxx|118xxx|100|155|195|09xx xxx xxxx|00xxx.|xx.|+xx.|(Mipd)|[^*#]@@.)
It's very comprehensive covering pretty much any possible number that may be dialled in the UK. Personally, I use a cut-down version:
Physical Interfaces > LINE Port > DigitMap:
(999|112|101|111|0[4568]xx.S3|0[123]xxxxxxxxx|07[1-9]xxxxxxxx|116xxx|xx.S3)
QuoteInstall any softphone on your mobile that supports SIP.
You would have to define a dummy SIP trunk on the OBi110 and bridge the call to the IP address of your mobile.
I use this method myself to allow incoming and outgoing calls via wifi to my mobile phone at home. However, I'm using an android phone and the SIP client CSipSimple, which only works with android. Two conditions need to be met to set this up:
1. Your router must be able to assign static subnet ip addresses to the OBi and SIP client on your mobile phone.
2. The SIP client must be able to call without registration.
If those two conditions are met, then you can use your mobile in the same way as a fixed phone plugged into the OBi.