Allow me to extend your pain. [by now i assume you have trawled through the commnity forum about issues with HG659 series modems]
You can't use a 3rd party modem, and get the vodafone sip settings from them. Vodafone will just not give it to you.
Even if u asked nicely, if they could assist you with setting voip on your own supllied modem, you will get a nice response,
1) VF SIP is only via the VF provided modem
2) stating vodafone engineers /staff are not trained to support a variety of modems nor to do anything more than basic modem and WiFi configuration.
The primary reason being, vodafone wants to avoid headaches with dealing with increased customer call volumes having voip settings issues
secondly, they mamaged theese modems centrally, yes they can push the settings and config chnages to these modems online. [yes these modems can talk back to command central on a management port]
Your options are:
1) Ditch Vodafone.Seek an Alternative.
2) ISP: MyRepublic. VoIP on VDSL or FIbre, voip settings are easy as. I am with my Republic and have own modem: Fritzbox 7490. loving it and never look back. love fritzbox so much, that have a second spare as well..
3) ISP: Stuff Fibre. Fibreplans only. [no adsl, vdsl]. landline is via an app. yes, you can have upto 5 devices with an.
Imagine you are at work and or at your mates place. you are connected to wifi. you receive landline calls on your app/ mobile phone. bloddy awesome. [i will move to stuff fibre, once fibre on my street]
4) voip Provider: 2talk.co.nz [check them out, you can have alternative voip landlines with them.
5) Spark. [they also use HG659/b series modems], but u can replace the modem with 3rdparty modem. no idea about heir voip settings, but if landline on fibre, it'll be straight plugged into Fibre ONT POTS port.
Worst case: vodafone may suggest you have your modem in bridge mode, whats the point.
** Some people have managed to retirve the voip settings from the xml file in HG 659 modem. thats great news, but what if vodafone chnages your voip password afterwards, you'll be in deep mud/catch 22 situation.