Converting GSM Calls to SIP with OpenWRT Using a 3G Modem and Asterisk

By HKL, on Friday 2024-12-13 20:42, tagged: 🏷️Networking 🏷️Operating

This article demonstrates how to convert GSM calls to SIP calls on an OpenWRT system using a Huawei 3G Modem and the Asterisk suite.

Translated by ChatGPT as the original article is in Chinese.

Installing Asterisk16 on OpenWRT

opkg update
opkg install asterisk16-app-system asterisk16-chan-dongle asterisk16-pjsip asterisk16-codec-ulaw asterisk16-codec-alaw asterisk16-res-rtp-asterisk asterisk16-bridge-simple

Configuring PJSIP as the Default Service and Adding Accounts

Modify /etc/asterisk/pjsip.conf:

[transport-udp]
type=transport
protocol=udp
bind=0.0.0.0

[6003]
type=endpoint
transport=transport-udp
context=from-internal
disallow=all
allow=ulaw
auth=6003-auth
aors=6003

[6003-auth]
type=auth
auth_type=userpass
password=6003
username=6003

[6003]
type=aor
max_contacts=1

[6004]
type=endpoint
transport=transport-udp
context=from-internal
disallow=all
allow=ulaw
auth=6004-auth
aors=6004

[6004-auth]
type=auth
auth_type=userpass
password=6004
username=6004

[6004]
type=aor
max_contacts=1

Adding an Internal Dial Plan

Modify /etc/asterisk/extensions.conf:

[from-internal]
exten => _Z.,1,Dial(PJSIP/${EXTEN},60,Trg)
same => n,Hangup()

Check extension status during calls:

asterisk -rvvvv
OpenWrt*CLI> pjsip show contacts

Using IAX for Extensions

For simplified NAT traversal, consider IAX extensions. Install and configure as follows:

opkg update
opkg install asterisk16-chan-iax2

Modify /etc/asterisk/iax.conf:

[general]
bindport=4569
bindaddr=0.0.0.0
iaxcompat=yes
nochecksums=yes
disallow=all
allow=ulaw

[6010]
type=friend
username=6010
secret=6010
context=from-internal
host=dynamic
callerid=6010<6010>

Update /etc/asterisk/extensions.conf:

[from-internal]
exten => 6010,1,Dial(IAX2/6010,60,Trg)

Configuring the Dongle Device

Configure /etc/asterisk/dongle.conf (adjust details as per your device):

[general]
interval=20

[defaults]
context=dongle-in
group=0
exten=+862022221234

[dongle0]
imei=123451234512345

Check device status:

asterisk -rvvvv
OpenWrt*CLI> pjsip show contacts

  Contact:  <Aor/ContactUri..............................> <Hash....> <Status> <RTT(ms)..>
==========================================================================================

  Contact:  6001/sip:6001@192.168.234.127:53117;transport= 378e2db08b NonQual         nan
  Contact:  6001/sip:6001@192.168.234.127:53117;transport= 378e2db08b NonQual         nan
  Contact:  6003/sip:6003@192.168.234.158:49989;transport= 2ec100f865 NonQual         nan
  Contact:  6003/sip:6003@192.168.234.158:49989;transport= 2ec100f865 NonQual         nan
  Contact:  6004/sip:6004@192.168.104.11:5060              586381001a NonQual         nan
  Contact:  6004/sip:6004@192.168.104.11:5060              586381001a NonQual         nan

Objects found: 6

    -- PJSIP/6001-00000005 is ringing
    -- PJSIP/6001-00000005 is ringing
       > 0x2262b00 -- Strict RTP learning after remote address set to: 192.168.234.127:52518
    -- PJSIP/6001-00000005 answered PJSIP/6004-00000004
       > 0x2270c60 -- Strict RTP learning after remote address set to: 192.168.104.11:11866
    -- Channel PJSIP/6001-00000005 joined 'simple_bridge' basic-bridge <ee120657-8627-4868-b677-cb0d896a2b5a>
    -- Channel PJSIP/6004-00000004 joined 'simple_bridge' basic-bridge <ee120657-8627-4868-b677-cb0d896a2b5a>
       > 0x2262b00 -- Strict RTP switching to RTP target address 192.168.234.127:52518 as source
       > 0x2270c60 -- Strict RTP switching to RTP target address 192.168.104.11:11866 as source
OpenWrt*CLI> pjsip show endpoints

 Endpoint:  <Endpoint/CID.....................................>  <State.....>  <Channels.>
    I/OAuth:  <AuthId/UserName...........................................................>
        Aor:  <Aor............................................>  <MaxContact>
      Contact:  <Aor/ContactUri..........................> <Hash....> <Status> <RTT(ms)..>
  Transport:  <TransportId........>  <Type>  <cos>  <tos>  <BindAddress..................>
   Identify:  <Identify/Endpoint.........................................................>
        Match:  <criteria.........................>
    Channel:  <ChannelId......................................>  <State.....>  <Time.....>
        Exten: <DialedExten...........>  CLCID: <ConnectedLineCID.......>
==========================================================================================

 Endpoint:  6001                                                 In use        1 of inf
     InAuth:  6001-auth/6001
        Aor:  6001                                               1
      Contact:  6001/sip:6001@192.168.234.127:53117;transp 378e2db08b NonQual         nan
  Transport:  transport-udp             udp      0      0  0.0.0.0:5060
    Channel: PJSIP/6001-00000005/AppDial                         Up            00:00:04   
        Exten:                           CLCID: "6004" <6004>

 Endpoint:  6002                                                 Unavailable   0 of inf
     InAuth:  6002-auth/6002
        Aor:  6002                                               1
  Transport:  transport-udp             udp      0      0  0.0.0.0:5060

 Endpoint:  6003                                                 Not in use    0 of inf
     InAuth:  6003-auth/6003
        Aor:  6003                                               1
      Contact:  6003/sip:6003@192.168.234.158:49989;transp 2ec100f865 NonQual         nan
  Transport:  transport-udp             udp      0      0  0.0.0.0:5060

 Endpoint:  6004                                                 In use        1 of inf
     InAuth:  6004-auth/6004
        Aor:  6004                                               1
      Contact:  6004/sip:6004@192.168.104.11:5060          586381001a NonQual         nan
  Transport:  transport-udp             udp      0      0  0.0.0.0:5060
    Channel: PJSIP/6004-00000004/Dial                            Up            00:00:04   
        Exten: 6001                      CLCID: "" <6001>


Objects found: 4

       > 0x2270c60 -- Strict RTP learning complete - Locking on source address 192.168.104.11:11866
       > 0x2262b00 -- Strict RTP learning complete - Locking on source address 192.168.234.127:52518

Adding Outgoing and Incoming Call Configurations

Update /etc/asterisk/extensions.conf:

[from-internal]
exten => _1.,1,Dial(Dongle/g0/${EXTEN:1})

[dongle-in]
exten => +862022221234,1,Dial(IAX2/6010,60,Trg)

Testing

Incoming Call Test

Incoming Test 1

Incoming Test 2

Outgoing Call Test

Outgoing Test 1

Outgoing Test 2