T3 to E3 conversions

We buy our transatlantic stuff with T3 ports either end, all our domestic
private circuits as T3s, and everything else internationally is carried
on circuits which use SDH and the "T" hierarchy. May be I'm unusual in
this respect, but I know people who have bought from BT and got T3s too.

In places like 60 Hudson St or 111 8th Av, or anywhere at all
in Europe that Colt or MFS (as examples) go, you should push for
a full VC3 rather than just a T3 or E3, assuming you have a way of
using it.

Elsewhere you will more likely be stuck with T3 or E3.

No, I don't know why at the E1/T1 level E1 is the preferred way of doing it.

It's historical.

Terminating E1s in D.C. (or anywhere else in the US beyond
the switches talking to the cables) was a mess involving special
gear. Terminating T1s in Europe "couldn't be done".

These days, since there are the VT1.5 and VT2 in SONET land
and the TU-11 and TU-12 in SDH land, the "standards gap"
is fixed by digital cross-connection; the choice of T1 vs
E1 payload is mostly down to who has the right trib cards,
and who has the most PDH cruft in the way, or (cynically)
down to E1s likely bringing in more revenue.