SIP ALG: the well-intentioned feature that breaks your VoIP
VoIP calls dropping, no audio, or some calls work and others don't? The culprit is often SIP ALG — an overlooked feature activated in your router.
Your VoIP calls are dropping? No audio? Or some calls work… and others don’t?
The problem doesn’t necessarily come from your operator. In many cases, the culprit is a little-known feature: SIP ALG.
SIP ALG: what is it for (in theory)?
SIP ALG is a feature found in certain routers and firewalls. Its goal is simple: to “help” VoIP work behind a private network (NAT).
To do this, it:
- modifies certain network exchanges
- adapts IP addresses
- dynamically opens ports
On paper, it sounds useful.
In practice: it creates more problems than it solves
Today, modern VoIP systems can handle these situations on their own. Result: SIP ALG becomes an extra layer… that disrupts everything.
Typical problems caused by SIP ALG
No audio during the call The call starts… but you hear nothing, or only one side of the conversation.
Unstable or random calls One call works, the next doesn’t. Without any apparent logic.
Drops after a few seconds Some calls drop after 30 seconds or 1 minute.
Works on 4G but not at the office A very good indicator of a local network issue — often SIP ALG.
Why does it happen?
Because SIP ALG modifies sensitive network information: IP addresses, communication ports, audio streams. And these modifications are not always consistent.
Result: the communication between your phone and the VoIP server becomes unstable.
Why is it common in businesses?
SIP ALG is often activated by default in standard routers, certain firewalls, and non-optimised network installations — without anyone knowing.
The key takeaway
SIP ALG is supposed to help VoIP… but today, it disrupts it more than anything else.
The MaxiConnect approach
At MaxiConnect, we see it regularly: a large proportion of VoIP issues don’t come from the operator’s network, but from the local configuration.
That’s why we prioritise:
- clean network configurations
- controlled infrastructures
- real technical support
Conclusion
If your VoIP telephony isn’t working correctly, the cause is very likely one single option: SIP ALG.
“In VoIP, the problem isn’t always what’s missing… sometimes it’s what’s been turned on by mistake.”