3.7 PSTN Trunks

Trunk is a term used to describe the connection to the Service Provider as opposed to the connection to the User Phone which is called a "Line". There are many kinds of trunks. A PSTN trunk can be one DS0, an OC192, and up! A PSTN trunk can also have multiple connections. For example, a single trunk can have 2,3 or more DS0 connections.

The muPBX utilizes two kinds of trunks: SIP Trunks and PSTN Trunks.

PSTN Trunks

PSTN (POTS) Trunks are necessary for localized services such as 0, 311, 411, 711 and 911. Some VoIP Service Providers offer 911 service but require you to register the location of all your muPBX phones, see 911 for details. POTS lines are also powered by the Local Service Provider's battery and will remain active in the event of a power outage. Another feature of the POTS line(s) is that faxing is more reliable than Fax over IP (FoIP).

There is no simple way to discuss this and it might get a little confusing but, try to stay with me!

You buy a PSTN (POTS) line from your Local Service Provider (SWB, AT&T, Verizon). To the Local Service Provider, you are their client and they expect you to install a phone (FXS device) on their POTS line (FXO device). An FXO will ONLY communicate with an FXS and vice-verse. So, when you connect a muPBX or an ATA to the POTS line, the muPBX or ATA must act like a FXS device. Note - Currently, I only recommend using an ATA to connect to the POTS line so we'll skip the discussion of muPBX FXS/FXO connections for now.

ATA Trunks

Most of the ATA devices I've seen want you to connect the FXO port to the POTS line. This is contrary to what was just discussed above and causes much confusion. Be certain you read the instruction on any device that connects to the POTS line or you'll be debugging it forever!

Below are some screen captures of an ATA FXO port configuration.

Here is the Trunk configuration to the ATA FXO.

host =
port = 5062
username = 6203
secret = 6203
trunkname = FXO1  ; GUI metadata
context = DID_trunk_1
group = null
hasexten = no
hasiax = no
hassip = yes
registeriax = no
registersip = no
trunkstyle = voip
insecure = port,invite
disallow = all
allow = ulaw,alaw,gsm,g726

Menu [toggle]