On a DSL line, you are sharing the bandwith with many other homes and users.
Most times, you could get the 2mb, but your real speed, moment to moment, is depending on what else the other users are doing.
Also, on DSL, your upload speed to the net is normally a fraction of your download speed.
Normal number will be 2mb download, 120 or 360 kb upload.
When you have a E1, you are one step closer to the real internet.
You get a real, not shared 2mb to a provider, and most imes you get the real 2mb regardless of what the teenager next door is doing on his/her computer.
Ans you will be able to send 2mb of data the other way too.
In VOIP ,if you have a 10 mb download, and 120 kb upload, you only have 120 kb of usable bandwith for VOIP.
Is a lot better to have 512 kb both ways trought a fractional E1 that to have a 100 mb download and 120 kb upload.