Проверял на Asterisk 11.7.0, Asterisk 11.14.1.
Пауза происходит между строчками
channel.c: Scheduling timer at (50 requested / 50 actual) timer ticks per second
и
channel.c: Auto-deactivating generator.
[2015-05-11 14:18:50] VERBOSE[8513][C-00000009] pbx.c: -- Executing [receive@fax-rx:16 ...

