Hi,
First of all terminology, "VoIP" voice over IP is a fairly generic term covering all sorts of protocols and architectures. SIP is one specific protocol.
A typical home VoIP system will use SIP and consist of a service you subscribe to, and your equipment. You configure your equipment to register with your chosen service, then it can make and receive calls on that service. Your equipment could be in the form of an adapter into which you plug a conventional telephone, or a standalone SIP device that connects into your network. I would normally recommend that latter, and that's what I use. I chose the Gigaset device because it could use the same DECT cordless handsets that I already use.
On your specifics, just because the router they've provided has a voice capability doesn't necessarily mean that the ISP provides that service, in fact if they're reluctant to give you access into that part of the router it suggests that maybe they don't.
Really it depends on what you're trying to achieve. If you want to play around with VoIP and SIP, there are various free or pay as you go services available, for example SIPgate which is what I use.