Cisco IP phones need what's called a "Call Manager" in order for them to function. The Call manager is either a dedicated server or it could be part of a router with a feature otherwise known as Call Manager Express.
So what you need to do is plug your Cisco IP Phone into the Cisco router at the very least and then possibly plug your Cisco router into your DSL or depending on the type of router it is, replace your DSL router all together (much like Curlyben indicated). I suspect that your Cisco router that was sent with your Cisco IP Phone is preconfigured to run a VPN back to your home office via your existing DSL connection.
So your Cisco IP Phone when plugged into your router will request a DHCP address which your Cisco router will provide. Along with the DHCP address the router also sends a DHCP Option 51 which tells the Phone where to get its configuration from as well as it also tell it who to contact for call routing setup which in this case is the Cisco router (the Call manager Express feature). After that, your router will know where the actual IP-to-PBX gateway is located in order to establish the call. So if your employer has the voice gateway accessible from the internet then your router will just route the voice traffic through your DSL router and on to your employers gateway. If your employers voice gateway is internal to your enterprise then your Cisco Router should establish a VPN connection back to your Enterprise.
Depending on the Cisco IP Phone you have you may have one or two jacks in the back of the phone. If you only have one jack then think of that jack like any other PC with an ethernet jack. If you have a 7960 phone then you'll have two jacks in the back. One goes to the router/switch and the other you plug a PC in. The ports on the back of the Cisco IP Phone should be clearly labeled.