[asterisk-users] Failed to authenticate device "Ext 110"

[asterisk users]

I'm having a strange problem recently with a Yealink SIP-T28P phone
connected to Asterisk 11.4.0 via openvpn.  It was working fine for months,
and now when I dial anything from the phone, it shows "Forbidden", and the
Asterisk console shows:

[May 21 10:47:49] NOTICE[28518][C-00000004]: chan_sip.c:25189
handle_request_invite: Failed to authenticate device "Ext 110" <
sip:110 at 192.168.6.2>;tag=1130259112

Asterisk 192.168.6.2
OpenVPN on router 10.8.0.1
Remote Yealink phone 10.8.0.6

The remote phone shows as being registered:
PBX*CLI> sip show peers
Name/username  Host  Dyn Forcerport ACL Port     Status      Description
110/110   10.8.0.6  D   A  5062   OK (111 ms) Yealink OpenVPN

Also, if there is voicemail in the mailbox for 110, the phone's message
light is lit and it beeps periodically.

toshi*CLI> sip show peer 110


  * Name       : 110
  Description  : Yealink OpenVPN
  Secret       : <Set>
  MD5Secret    : <Not set>
  Remote Secret: <Not set>
  Context      : remote-phones
  Record On feature : automon
  Record Off feature : automon
  Subscr.Cont. : <Not set>
  Language     :
  Tonezone     : <Not set>
  AMA flags    : Unknown
  Transfer mode: open
  CallingPres  : Presentation Allowed, Not Screened
  Callgroup    : 1
  Pickupgroup  : 1
  Named Callgr :
  Nam. Pickupgr:
  MOH Suggest  :
  Mailbox      : 110
  VM Extension : asterisk
  LastMsgsSent : 1/0
  Call limit   : 4
  Max forwards : 0
  Dynamic      : Yes
  Callerid     : "Ext 110" <110>
  MaxCallBR    : 384 kbps
  Expire       : 608
  Insecure     : no
  Force rport  : No
  Symmetric RTP: No
  ACL          : Yes
  DirectMedACL : No
  T.38 support : No
  T.38 EC mode : Unknown
  T.38 MaxDtgrm: -1
  DirectMedia  : No
  PromiscRedir : No
  User=Phone   : No
  Video Support: Yes
  Text Support : No
  Ign SDP ver  : No
  Trust RPID   : Yes
  Send RPID    : Yes
  Subscriptions: Yes
  Overlap dial : No
  DTMFmode     : rfc2833
  Timer T1     : 500
  Timer B      : 32000
  ToHost       :
  Addr->IP     : 10.8.0.6:5062
  Defaddr->IP  : 10.8.0.6:5060
  Prim.Transp. : UDP
  Allowed.Trsp : UDP
  Def. Username: 110
  SIP Options  : (none)
  Codecs       : (ulaw)
  Codec Order  : (ulaw:20)
  Auto-Framing :  No
  Status       : OK (237 ms)
  Useragent    : Yealink SIP-T28P 2.61.23.3 00:15:65:xx.xx.xx
  Reg. Contact : sip:110 at 10.8.0.6:5062
  Qualify Freq : 60000 ms
  Keepalive    : 0 ms
  Sess-Timers  : Accept
  Sess-Refresh : uas
  Sess-Expires : 1800 secs
  Min-Sess     : 90 secs
  RTP Engine   : asterisk
  Parkinglot   :
  Use Reason   : No
  Encryption   : No

sip.conf:

[110]
context=remote-phones
type=peer
host=dynamic
qualify=1500
canreinvite=no
dtmfmode=rfc2833
progressinband=no
callgroup=1
pickupgroup=1           ; We can do call pickup for call group 1
call-limit=4
busy-level=1
qualify=yes
deny=0.0.0.0/0.0.0.0
permit=0.0.0.0/0.0.0.0
nat=no
qualify=8000
description=Yealink OpenVPN
defaultuser=110
secret=xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
callerid="Ext 110" <110>
mailbox=110
defaultip=10.8.0.6
port=5060
disallow=all
allow=ulaw

Any suggestions on what might be happening here, and how it could be
resolved?

THANKS ALL!
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