Unable to merge calls

I have the same issue. The merge button is there but when you press it nothing happens. I called my network provider (O2) to ask them to activate it and they claim they don't need to, that it's a device option, not something the network provider has to authorise. I'm really in a hurry to get this working as I have to record a phone call tomorrow afternoon and the app I've downloaded to do this requires that you be able to merge calls...