New T1 line - What am I missing?

We had a T1 line installed by ATT to replace several analog phone lines and DSL internet service.

As I understand it, this is what I have:

"Smart Jack," the connection point from ATT to me.

"IPFlex router," the device that translates and routes VoIP to my Nortel BCM50 "PBX" system and data to my LAN. (Specifically, from the ATT equipment order list: "4608T4 Edgemarc 75 Network Services Gateway Quad T1 enabled with WAP, Single PRI with DSP and 12 port PoE managed switch.)

All I was told to provide was a CAT5 cable from my phone room to my network. Done.

After installation, I assumed that all I had to do was connect the network cable to the "router" they installed and my network switch (that's what I had done with the ATT-provided DSL router). Nothing.

I went out and bought a simple 150 Mbs router and put it in between the new "router" and my network switch. That worked, but without the promised T1 speed.

I am fairly certain that all of my computers in the office network are all connected with CAT5 cable through 10/100 NICs; so I don't think that my office hardware is the bottleneck.

What am I missing here?

Is the "IPFlex router" (Edgemarc blah, blah, blah) a router the way I understand a router (as in, like my previous DSL router)? (I tend to think that it is not, since it does not provide a firewall.)

Is the additional router I purchased extraneous or just plain wrong? That is, is it in the way or is it the wrong piece of equipment?

Thanks, in advance, for your help.