Heaps of thanks David, and apologies for my lack of certainty regarding what you mean by "trace".

I have increased debugging on all aspects and seem have turned up the following entries from the log and thought I would ask for a sanity check here.
Does the context look okay to you? 0223334444 is an alias for my mobile number.

I can see where certain decisions are being made such as responding to my INVITE with 480 at the end, but why? Is there another log I can set up or something?

...
2017-07-06 16:10:40.694638 [DEBUG] mod_sofia.c:143 sofia/internal/200@192.168.1.13 SOFIA ROUTING
2017-07-06 16:10:40.694638 [DEBUG] switch_core_state_machine.c:236 sofia/internal/200@192.168.1.13 Standard ROUTING
2017-07-06 16:10:40.694638 [INFO] mod_dialplan_xml.c:637 Processing 200 <200>->0223334444 in context 192.168.1.13
Dialplan: sofia/internal/200@192.168.1.13 parsing [192.168.1.13->user_exists] continue=true
Dialplan: sofia/internal/200@192.168.1.13 Regex (PASS) [user_exists] () =~ // break=on-false
Dialplan: sofia/internal/200@192.168.1.13 Action set(user_exists=${user_exists id ${destination_number} ${domain_name}}) INLINE
2017-07-06 16:10:40.733859 [DEBUG] freeswitch_lua.cpp:365 DBH handle 0x7f2ba40aa750 Connected.
2017-07-06 16:10:40.733859 [DEBUG] freeswitch_lua.cpp:382 DBH handle 0x7f2ba40aa750 released.
EXECUTE sofia/internal/200@192.168.1.13 set(user_exists=false)
2017-07-06 16:10:40.733859 [DEBUG] mod_dptools.c:1530 SET sofia/internal/200@192.168.1.13 [user_exists]=[false]
...

///Note: after a whole lot of conditionals and such, things appear to get interesting again:
...
2017-07-06 16:10:40.753859 [DEBUG] mod_sofia.c:198 sofia/internal/200@192.168.1.13 SOFIA EXECUTE
2017-07-06 16:10:40.753859 [DEBUG] switch_core_state_machine.c:328 sofia/internal/200@192.168.1.13 Standard EXECUTE
ed34877a-2bed-4478-947a-13bc01ef90a7 EXECUTE sofia/internal/200@192.168.1.13 set(call_direction=local)
2017-07-06 16:10:40.753859 [DEBUG] mod_dptools.c:1530 SET sofia/internal/200@192.168.1.13 [call_direction]=[local]
EXECUTE sofia/internal/200@192.168.1.13 export(origination_callee_id_name=0223334444)
2017-07-06 16:10:40.753859 [DEBUG] switch_channel.c:1296 EXPORT (export_vars) [origination_callee_id_name]=[0223334444]
EXECUTE sofia/internal/200@192.168.1.13 set(RFC2822_DATE=Thu, 06 Jul 2017 16:10:40 +1200)
2017-07-06 16:10:40.753859 [DEBUG] mod_dptools.c:1530 SET sofia/internal/200@192.168.1.13 [RFC2822_DATE]=[Thu, 06 Jul 2017 16:10:40 +1200] 34877a-2bed-4478-947a-13bc01ef90a7 EXECUTE sofia/internal/200@192.168.1.13 hash(insert/192.168.1.13-last_dial/200/0226391300)
ed34877a-2bed-4478-947a-13bc01ef90a7 EXECUTE sofia/internal/200@192.168.1.13 eval(not_secure)
2017-07-06 16:10:40.753859 [NOTICE] switch_core_state_machine.c:385 sofia/internal/200@192.168.1.13 has executed the last dialplan instruction, hanging up.
2017-07-06 16:10:40.753859 [NOTICE] switch_core_state_machine.c:387 Hangup sofia/internal/200@192.168.1.13 [CS_EXECUTE] [NORMAL_CLEARING]
2017-07-06 16:10:40.753859 [DEBUG] switch_core_state_machine.c:650 (sofia/internal/200@192.168.1.13) State EXECUTE going to sleep
2017-07-06 16:10:40.753859 [DEBUG] switch_core_state_machine.c:584 (sofia/internal/200@192.168.1.13) Running State Change CS_HANGUP (Cur 1 Tot 15)
2017-07-06 16:10:40.753859 [DEBUG] switch_core_state_machine.c:850 (sofia/internal/200@192.168.1.13) Callstate Change RINGING -> HANGUP
2017-07-06 16:10:40.753859 [DEBUG] switch_core_state_machine.c:852 (sofia/internal/200@192.168.1.13) State HANGUP
2017-07-06 16:10:40.753859 [DEBUG] mod_sofia.c:438 Channel sofia/internal/200@192.168.1.13 hanging up, cause: NORMAL_CLEARING
2017-07-06 16:10:40.753859 [DEBUG] mod_sofia.c:577 Responding to INVITE with: 480

Many many thanks for looking this over!

Nate.

On Thu, Jul 6, 2017 at 12:07 AM, David Villasmil <david.villasmil.work@gmail.com> wrote:

Fs will fallback to Digest from ACL when the source ip is not allowed, this is normal.

Looking at your log, it doesn't seem like the client is actually responding to the challenge.

Can you take a trace and look at it? You should see the challenge and then an INVITE with credentials.