BT:
-----
#0  0xa7fc7024 in pthread_mutex_lock () from /lib/tls/i686/cmov/libpthread.so.0
#1  0xa77bcd31 in local_fixup (oldchan=0x843b340, newchan=0x83249e0) at ../include/asterisk/lock.h:601
#2  0x08064451 in ast_do_masquerade (original=0x83249e0) at channel.c:3154
#3  0x08089389 in ast_async_goto (chan=0x843b340, context=0x849146c "intern", exten=0x84910c0 "60", priority=1) at pbx.c:4583
#4  0xa783b40c in handle_request_refer (p=0x8490eb8, req=0xa77fcb08, debug=<value optimized out>, ignore=0, seqno=4, nounlock=0xa77fdea4) at chan_sip.c:10888
#5  0xa783c49c in handle_request (p=0x8490eb8, req=0xa77fcb08, sin=0xa77fde90, recount=0xa77fdea0, nounlock=0xa77fdea4) at chan_sip.c:11390
#6  0xa783e56e in sipsock_read (id=0x8168490, fd=14, events=1, ignore=0x0) at chan_sip.c:11538
#7  0x08055dce in ast_io_wait (ioc=0x8168160, howlong=527) at io.c:284
#8  0xa7821bbb in do_monitor (data=0x0) at chan_sip.c:11701
#9  0xa7fc5240 in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#10 0xa7ea132e in clone () from /lib/tls/i686/cmov/libc.so.6

BTFULL:
---------
No symbol table info available.
#1  0xa77bcd31 in local_fixup (oldchan=0x843b340, newchan=0x83249e0) at ../include/asterisk/lock.h:601
        __PRETTY_FUNCTION__ = "local_fixup"
#2  0x08064451 in ast_do_masquerade (original=0x83249e0) at channel.c:3154
        x = 8
        i = <value optimized out>
        res = <value optimized out>
        origstate = <value optimized out>
        cur = <value optimized out>
        t = <value optimized out>
        t_pvt = <value optimized out>
        tmpcid = {cid_dnid = 0x0, cid_num = 0x0, cid_name = 0x0, cid_ani = 0x0, cid_rdnis = 0x0, cid_pres = 0, cid_ani2 = 0, cid_ton = 0, cid_tns = 0}
        clone = (struct ast_channel *) 0x843b340
        spy_list = <value optimized out>
        spy = <value optimized out>
        rformat = 64
        wformat = 64
        newn = "Local/600@intern-355c,1<ZOMBIE>", '\0' <repeats 21 times>, "\027pü§\f´C\b", '\0' <repeats 12 times>, "\001\000\000\000\f´C\b\000\000\000\000¬J2\b\032\v\000\000E4\006\b¬J2\b"
        orig = "AsyncGoto/Local/600@intern-355c,1<ZOMBIE>\000\000\000\000\000\000\000\210d7\b\001å\000\000\000\213O\bp\000\000\000\000\000\000\000L\236\177§\027pü§@³C\b_\221\005\b\001\000\000\000p\000\000\000@³C\b\b\236\177§"
        masqn = "Local/600@intern-355c,1<ZOMBIE><MASQ>", '\0' <repeats 35 times>, "\200d7\b\bd7\b\017\000\000\000\000\000\000\0008\002\000\000\000\000\000\000\000\000\000"
        zombn = "AsyncGoto/Local/600@intern-355c,1<ZOMBIE><ZOMBIE>", '\0' <repeats 11 times>, "¤¸ã§", '\0' <repeats 28 times>, "\002\000\000\000øHð§"
        __PRETTY_FUNCTION__ = "ast_do_masquerade"
#3  0x08089389 in ast_async_goto (chan=0x843b340, context=0x849146c "intern", exten=0x84910c0 "60", priority=1) at pbx.c:4583
        tmpchan = (struct ast_channel *) 0x83249e0
        res = 0
        __PRETTY_FUNCTION__ = "ast_async_goto"
#4  0xa783b40c in handle_request_refer (p=0x8490eb8, req=0xa77fcb08, debug=<value optimized out>, ignore=0, seqno=4, nounlock=0xa77fdea4) at chan_sip.c:10888
        nobye = 0
        c = (struct ast_channel *) 0x843ab20
        res = 0
        transfer_to = (struct ast_channel *) 0x843b340
        transfercontext = 0x849146c "intern"
        __PRETTY_FUNCTION__ = "handle_request_refer"
#5  0xa783c49c in handle_request (p=0x8490eb8, req=0xa77fcb08, sin=0xa77fde90, recount=0xa77fdea0, nounlock=0xa77fdea4) at chan_sip.c:11390
        resp = {rlPart1 = 0x0, rlPart2 = 0x0, len = 0, headers = 0, method = 0, header = {0x0 <repeats 64 times>}, lines = 0, line = {
    0x0 <repeats 64 times>}, data = '\0' <repeats 4095 times>, debug = 0, flags = 0, sdp_start = 0, sdp_end = 0}
        cmd = 0xa77fcd20 "REFER"
        cseq = <value optimized out>
        useragent = <value optimized out>
        seqno = 4
        len = <value optimized out>
        ignore = 0
        respid = <value optimized out>
        res = <value optimized out>
        iabuf = "&Í\177§\027pü§\000\000\000\000Î_\200§"
        debug = 0
        e = 0xa77fcd26 "sip:26@1.1.1.17"
        __PRETTY_FUNCTION__ = "handle_request"
#6  0xa783e56e in sipsock_read (id=0x8168490, fd=14, events=1, ignore=0x0) at chan_sip.c:11538
        tmp = "ô\224ï§!,\000\000\b\227O\bøØ\000\000X\225ï§\001\000\000\000»ó\200§\b\230\204§\234\234\204§\027pü§\000\000\000\000\rª\201§\234\234\204§ôD\204§7\005\000\000Ä\005\000\000\230©\025\b\027pü§ \203\030\b\223d\005\b"
        req = {rlPart1 = 0xa77fcd20 "REFER", rlPart2 = 0xa77fcd26 "sip:26@1.1.1.17", len = 494, headers = 12, method = 9, header = {0xa77fcd20 "REFER",
    0xa77fcd3f "Via: SIP/2.0/UDP 1.1.2.42:2051;branch=z9hG4bK-kut664g1dhdg;rport",
    0xa77fcd81 "From: \"Bastian XYZ\" <sip:test@1.1.1.17>;tag=07emhka14b", 0xa77fcdc2 "To: \"Nachtschaltung\" <asterisk@1.1.1.17>;tag=as6814602b",
    0xa77fcdfb "Call-ID: 3ccde1c71117-iyp367jnlj7g@snom320-00041324A82E", 0xa77fce34 "CSeq: 4 REFER", 0xa77fce43 "Max-Forwards: 70",
    0xa77fce55 "Contact: <sip:test@1.1.2.42:2051;line=3q666u5t>;flow-id=1", 0xa77fce94 "Refer-To: sip:60@1.1.1.17;user=phone",
    0xa77fceba "Referred-By: sip:test@1.1.1.17", 0xa77fcede "User-Agent: snom320/6.2.3", 0xa77fcef9 "Content-Length: 0", 0xa77fcf0c "",
    0x0 <repeats 51 times>}, lines = 0, line = {0xa77fcf0e "", 0x0 <repeats 63 times>},
  data = "REFER\000sip:26@1.1.1.17\000SIP/2.0\000\000Via: SIP/2.0/UDP 1.1.2.42:2051;branch=z9hG4bK-kut664g1dhdg;rport\000\000From: \"Bastian XYZ\" <sip:test@1.1.1.17>;tag=07emhka14b\000\000To: \"Nachtschaltung\" <asterisk@1.1.1.1"..., debug = 0, flags = 0, sdp_start = 0, sdp_end = 0}
        sin = {sin_family = 2, sin_port = 776, sin_addr = {s_addr = 704774401}, sin_zero = "\000\000\000\000\000\000\000"}
        p = (struct sip_pvt *) 0xa77bcd00
        res = <value optimized out>
        len = 16
        nounlock = 1
        recount = 0
        iabuf = "\000\000\000\000\032$&F\032$&F\000\000\000"
        lockretry = 100
        __PRETTY_FUNCTION__ = "sipsock_read"
#7  0x08055dce in ast_io_wait (ioc=0x8168160, howlong=527) at io.c:284
        res = 1
        x = 0
        origcnt = 1
#8  0xa7821bbb in do_monitor (data=0x0) at chan_sip.c:11701
        res = <value optimized out>
        sip = (struct sip_pvt *) 0x0
        t = 1176904730
        fastrestart = 0
        lastpeernum = -1
        curpeernum = 53
        reloading = <value optimized out>
        __PRETTY_FUNCTION__ = "do_monitor"
#9  0xa7fc5240 in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
No symbol table info available.
#10 0xa7ea132e in clone () from /lib/tls/i686/cmov/libc.so.6
No symbol table info available.