The 659 has an in-built capability to handle a VoIP account - all you needed to do was configure 2talk on the the modem/router itself and plug a normal phone into it.
It didn't work if you had VDSL though. ADSL and Fibre were ok. Huawei did get involved and I'm not 100% sure this is still a problem.
Even if you have this modem on VDSL, you could still get an ATA or a VoIP phone and use that as opposed to the in-built functionality.
Not a vodafone supplied HG659, the internal voip firmware has been disabled. Will only be enabled when you get a bundled connection with them.
The Vodafone supplied HG659 doesnt have its voip firmware disabled. I have one and I have 2talk working on it no problems.