Trunk-to-trunk transfer is available as a line based feature in CENTREX and on some PRI configurations, also on SUPERTRUNK(PB) on 5ESS switches. Recently SBC(PB) started offering a feature called 'call transfer disconnect' on single analogue lines, but I have not found out all the details as to 5ESS or DMS100, etc switches...
As a CENTREX feature it works great, I setup a PARTNER for a customer that needed to do this.
Hope this helps!
....JIM....