Best way to set up a 2 router network?

Greetings.

I have a somewhat unique networking situation which I'd like some help with. I'm in need of using a friends' network and need to connect my Vonage VOIP telephone to their network in the least obtrusive manner possible.

The current network configuration of my friends' network is this: They have a newer home that is pre-wired for internet connectivity with an Ethernet port coming directly from the wall. They have Comcast and the main cable connection comes into their house and is split to various tv/cable ports. One of these splits goes immediately (in a networking panel in a closet) to a modem (in this same closet) and the Ethernet cable comes out of the modem which then gives the connectivity to the Ethernet port which is in the wall. They have a wireless router attached directly to the Ethernet line coming from their wall which is broadcasting for a few wireless devices and also is connected to one wired desktop. Pretty straight forward at this point. They have no VOIP router as their telephone is the old school style via a telephone wire.

Enter me. I need to hook up my Motorola VT2542 VOIP device (using Vonage) to their network so that I can have my own telephone number at their house. This device is also a wireless router. I do not need to use the VT2542 for any internet connectivity though, only for the phone line. 'm basically trying to run two routers out of one modem - one for internet connectivity and the other for my telephone. What's the best way to do this? I

Right now I am running it out of their wireless router (which is basically connecting two routers together). Although this works, it gives my telephone line a great deal of static because I'm running a router through another router. Is there a better way to do this? Do I need an Ethernet switch? or hub?

Any help would truly be appreciated.
Thank you.

 

Well, due to time constraints I just ended up swapping out their router with mine and temporarily reconfiguring their network. It wasn't too big of a deal and was pretty unobtrusive thankfully. When I chained the routers together my VOIP phone line became too static-y, which makes sense.

So basically I did a workaround.

 

in order to prevent cross-talk, both wireless routers must be on a different channel