What Is a VoIP Gateway? FXS vs FXO Explained
A VoIP Gateway is a hardware appliance that connects traditional telephony (analog, ISDN, GSM) to an IP voice network (SIP). It lets organizations migrate legacy phone systems to VoIP without ripping everything out at once. This guide covers FXS, FXO, GSM Gateway, E1 Gateway, and selection criteria.
What Is a VoIP Gateway
A VoIP (Voice over IP) Gateway converts audio between traditional telephony (analog, ISDN, GSM) and IP networks (SIP, RTP). It enables legacy phone systems to interoperate with IP PBX, Cloud PBX, or Microsoft Teams.
A VoIP Gateway has two sides: one offers analog ports (RJ-11), ISDN ports (RJ-45 BRI/PRI), or GSM modules; the other offers IP ports (RJ-45 Ethernet). The internal DSP converts audio to RTP packets and signals over SIP.
Types of VoIP Gateway
VoIP Gateways are categorized by interface type, with four common forms.
FXS Gateway
An FXS (Foreign Exchange Subscriber) Gateway "emulates" a carrier trunk, used to connect old analog phones, fax machines, or door phones into an IP PBX.
FXS ports supply -48V DC power, ring voltage, and dial tone to the connected device, just like a regular carrier line. Example: plug a fax machine into an FXS Gateway to send faxes over a VoIP network.
FXO Gateway
An FXO (Foreign Exchange Office) Gateway "receives" a carrier trunk, used to connect existing analog trunks (copper 02-XXX-XXXX lines) into an IP PBX.
FXO ports receive power and ring signals from the carrier and pass them to the IP PBX as SIP calls. Useful for organizations with legacy analog lines that aren't ready to migrate to SIP Trunks.
GSM / 4G Gateway
A GSM Gateway has SIM card slots used to connect to mobile carriers (AIS, True, dtac) for cheaper on-net mobile calls or as a backup when analog lines or SIP Trunks fail.
4G Gateways support Voice over LTE (VoLTE), delivering better audio than legacy GSM 2G/3G.
E1 / PRI Gateway
An E1 / PRI Gateway connects ISDN E1 (30 channels) trunks from a carrier to an IP PBX. Suited to large organizations with existing ISDN circuits that want to migrate to IP PBX gradually.
FXS vs FXO Difference
The most common confusion is when to use FXS vs FXO ports. Simple rule: FXS emulates the carrier (supplies power), FXO emulates the phone (receives power).
- FXS = carrier side: connect analog phones, faxes, door phones
- FXO = subscriber side: connect carrier trunks (copper lines)
- FXS Gateway: turns old analog phones into IP extensions
- FXO Gateway: brings existing analog trunks into the IP PBX
- Hybrid devices: Yeastar TG, Grandstream HT812 ship with both FXS and FXO
VoIP Gateway Use Cases
Even after IP PBX migration, gateways are still needed in several scenarios.
Legacy PBX Migration
Organizations with a 10-year-old PBX and copper 02 lines that don't want to port numbers immediately can use an FXO Gateway to bring the old lines into a new IP PBX, then port to SIP Trunks later.
Fax and Paging Systems
Fax machines, paging speakers, door phones, and elevator phones are usually still analog. FXS Gateways let them connect to an IP PBX without replacing the devices.
Call Centers Needing GSM Routing
Call centers that mainly call users on the same mobile network (AIS-AIS, True-True) can use GSM Gateways for much cheaper on-net minute rates.
Voice Backup During SIP Trunk Failure
Sites needing disaster recovery can use a GSM Gateway as a backup when internet or SIP Trunks fail, automatically failing over to 4G calling.
Choosing a VoIP Gateway
Evaluate five key factors before purchasing.
- Port count: 2, 4, 8, 16, 24, 32 ports based on use
- Port type: FXS, FXO, GSM, E1, or hybrid
- Codecs: G.711, G.722, G.729, Opus support
- SIP compatibility: works with your PBX (Yeastar, 3CX, Asterisk, Cisco)
- Features: echo cancellation, T.38 fax, caller ID, SRTP
Popular VoIP Gateway Brands in Thailand
Multiple brands span different price points and use cases.
- Yeastar TG / TA Series: SME-focused, supports FXS, FXO, GSM, E1
- Grandstream HT and GXW Series: budget-friendly, T.38 fax support
- AudioCodes Mediant: carrier-grade, E1 / T1 support, popular for Microsoft Teams Direct Routing
- Sangoma Vega Series: common in the Asterisk ecosystem, supports E1 / T1 / FXS / FXO
- Patton SmartNode: branch office gateways for ISDN and analog
Frequently Asked Questions
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