(gdb)  thread apply all bt
Thread 1 (process 14682):
#0  0x05adc311 in local_attended_transfer (transferer=0xb7dcea30, current=0x3942174, req=<value optimized out>, seqno=4)
    at chan_sip.c:18759
#1  0x05adde24 in handle_request_refer (p=0xb7dcea30, req=0x3945020, debug=<value optimized out>, seqno=4, nounlock=0x3944fd4)
    at chan_sip.c:18996
#2  0x05b1352b in handle_incoming (p=0xb7dcea30, req=0x3945020, sin=0x3946258, recount=0x3944fd8, nounlock=0x3944fd4)
    at chan_sip.c:19899
#3  0x05b14ee1 in handle_request_do (req=0x3945020, sin=0x3946258) at chan_sip.c:20174
#4  0x05b15a5e in sipsock_read (id=0xa041b80, fd=22, events=1, ignore=0x0) at chan_sip.c:20080
#5  0x080cf7c0 in ast_io_wait (ioc=0xa14ab50, howlong=1000) at io.c:288
#6  0x05aed4fd in do_monitor (data=0x0) at chan_sip.c:20575
#7  0x0813689b in dummy_start (data=0xa037938) at utils.c:861
#8  0x00c8f33a in start_thread () from /lib/libpthread.so.0
#9  0x03946470 in ?? ()
#10 0x03946470 in ?? ()
#11 0x03946470 in ?? ()
#12 0x03946470 in ?? ()
#13 0x00000000 in ?? ()