There is an adapter, it is known as a HiPort
But you can use your current Tmobile router as an adapter as well. Both of these items are very similar inside, with only the extra ports broken out from the chip inside the router. What you'd do is hook your new router WAN port to your cable or DSL modem, then plug the WAN port on your Tmobile router to a LAN port on your new router. Finally go into your Tmobile router settings and turn off wireless networking.
This should work fine as is. If it doesn't, the next thing to do is in your new router, assign the Tmobile router IP address to the DMZ. This means data from the internet to the Tmobile router (your calls) will cross the new router absolutely unmolested. There are many other setting tweaks you can do, but this should be enough to get you going.
I have a @home router which has a SIM card with the home phone number attached to it. I need to keep the home phone number on, and would like to keep it consolidated with my cell phone bill. My issue is the router is not strong enough to keep up with my networking needs! I have a new router, I can not find anything to put the SIM card in to keep the home phone on. I'm really looking for something like an adapter or something that would read the SIM card but some how plug into the new router. Is there a way to make the new router the primary wireless router and access point and still have the @home router connected to it?