Zen has VoIP for domestic users but is only for outgoing calls with the CLID being your landline number. May be OK for some but it is not the real deal. It costs £5.10 a month. I will be ditching this service soon.
Voipfone is OK and can be free if you are prepared to contemplate an 056 number; scale up to a regular 01/02 number and you have a monthly charge £3.60 (New price now in effect). Calls to 01/02 cost 1ppm (no longer charged by the second) but to UK mobile phones it is a staggering 9ppm.
There are call plans available but it would be unusual to actually save money using their offerings. On the plus side support is good and there are options available not found with other hosted providers. It is a steep learning curve to master their dashboard. Calls are free to other Voipfone customers.