Incubust by Terry Pratchett

The physics of magic is this: no magician, disguise it as he might, can achieve a result beyond his own physical powers.

And, spurned, he performed the Rite of Tumescence and called up a fiend from the depths of the Pit to teach her a lesson she wouldn’t forget, the witch.

The phone rang. “Nice try,” she said, “It’s sitting on the bedhead now.” His breath quickened. “And?”

“Listen,” she said. And he heard the voice of the fiend, distant and wretched:

“…frightfully sorry … normally, no problem … oh god, this has never happened to me before…”

See the Necrotelicomnicon, p.38.

1988 T. Pratchett