How SIP Works
SIP is a text-based protocol similar to email or HTTP. Each participant has a SIP address in the form of user@domain (for example 100@firma.cz). The SIP server, also known as a proxy or registrar, mediates the connection between the caller and the called party.
Call progress:
- The caller sends an SIP INVITE message to the server.
- The server locates the called party and forwards the invitation to them.
- Upon receipt (response 200 OK), both participants transmit voice directly via RTP.
- The end of the call is signalled by the message BYE.
SIP in practice for a company
SIP is the foundation of modern business telephony. It is used by softphones on computers, IP phones on desks, and cloud-based PBX exchanges. Because it is an open standard, devices and exchanges from different manufacturers work together without problems.
- Enables calls over the internet without dependence on a single operator.
- Supports call forwarding, hold, transfer and conference calls.
- SIP trunks replace traditional ISDN lines.
- It also works for video calls and short message transmission.
Safety and correct configuration of SIP
Unsecured SIP is a target for attacks, attackers attempt to crack passwords and misuse lines to call premium numbers (so-called toll fraud). For secure operation, it is important:
- Use TLS for encrypting signalling and SRTP for encrypting transmitted audio
- Set strong passwords for SIP accounts
- Restrict access to the SIP server to trusted IP addresses
- Ban SIP ALG on the company router: this function modifies SIP packets and causes call drops
On virtualni-ustredna.cz, security aspects of SIP are handled at the platform level without the need for manual configuration.
Frequently Asked Questions
Is SIP exactly the same as VoIP?
No. VoIP (Voice over IP) is a general term for voice transmission over the internet. SIP is a specific signalling protocol, one of several ways to implement VoIP. In practice, however, SIP is the most widely used standard.
What is SIP registration?
SIP registration is the process of registering a device (a phone or softphone) with a SIP server. Only after successful registration can the device receive incoming calls and place outgoing ones.
Do I need special internet for SIP?
No, SIP works on standard internet connections. For good call quality, low latency and low jitter are important, therefore, it is recommended to configure QoS (traffic prioritisation) on the company router.