Linksys Wireless Router :: Tmobile WRTU54G-TM Connection To VOIP Service?

A client is using Tmobile VOIP using the aforementioned WRTU54G-TM 2-line VOIP wireless router with 4-port hub.  What they would like to do is to place this in their network and NOT have it change (route) the network addresses being assigned to devices 'behind' this router so even their wireless devices are all "on the same network" as the wired devices BEFORE this unit,  In essence, preserve the same 'network and subnet mask' that is being issued by their DSL router via it's integrated DHCP server. ... Rather like operating this unit like an 'access point' instead of a router.Under most circumstances, I might try to assign this router a static IP outside the scope of the DHCP range the DSL router would supply, then plug the (gigabit) Ethernet network cable into one of the 4 Ethernet ports on the rear of this router - but apparently this does not 'facilitate' a connection to Tmobile's VOIP service.When attempting to do this, the connected telephone devices do not get a phone line connection and the status light(s) that normally indicate proper operation do not illuminate.