Hey haseeb36.
Dell only has recovery ISO's for ESXi 5.1 for systems that are on the HCL. Aside from that you have to build your own custome ISO for ESXi installation. I'm guessing your T20 has a nic that VMware doesn't have a driver for in the base image. What kind of NIC's does your T20 have in it.
Also when I did a search on VMwards Hardware Compatability List the PowerEdge T20 didn't show up, so its possible you may have problems or have to do more hackary to get a driver to work for that nic.
Let us know which NIC it is and we can see what we can do