Ulster's hopes of a home quarter-final were dashed on Sunday when Saracens claimed a bonus point win over Edinburgh at Vicarage Road.
The English Premiership side, coached by Ulster 1999 European Cup winner McCall, ran in five tries, four of them in the second half, in beating the Scots 40-7.
It looked like a home quarter-final could have been on the cards after Ulster recorded their first win in France at Stade Pierre-Antoine on Saturday.
They defeated Castres 9-8 in the scrappy Heineken Cup clash.
The Ulstermen came from behind to win, courtesy of three Ruan Pienaar penalties.
Castres had led early on thanks to a well-worked try from Piula Faasalele but it was Mark Anscombe's side who took home the points.
"We were very flat in the first half, we kept giving the ball away and couldn't get our lineout going," Anscombe said.
"We couldn't come here and expect to score four tries, but there were opportunities and holes in Castres. We just needed to take a bit more care with the ball."
"The character has been the hallmark of the side for the entire season so far, and we showed the strength of our character in the second half today," Anscombe added.
"It was good to tick that box and get that first win in France in the Heineken Cup."
Quarter final ties (5-7 April 2013):
- Harlequins v Munster,
- Clermont Auvergne v Montpellier,
- Toulon v Leicester,
- Saracens v Ulster