Have a look at Voipfone which is who I use, costs around £2 per month and then call divert to mobile works out around 12p per minute if memory serves me correct. What I do is leave a voice message saying asking them to leave a message as I may be driving or using machinery and might not hear the phone, that way your phone bill will not to high and then I call them straight back. I find 95% people will leave a message, its usually a sales call if no number or no message.