What Is TLS for SIP? Encrypted SIP Signalling Explained
Every time a phone registers, it sends credentials across the network. Send them in plain text and you've handed anyone listening the keys to your phone system. TLS is the difference between whispering and shouting your password.
TLS for SIP (SIP over TLS, usually on port 5061) encrypts the signalling of a call so credentials, phone numbers and call setup details can't be intercepted or altered. It's the signalling-side counterpart to SRTP's media encryption.
What TLS protects
Plain SIP sends everything — registration credentials, who is calling whom, call setup — in clear text. On an untrusted network, that's readable by anyone in the path, and credentials captured this way are a direct route to toll fraud. TLS wraps the SIP channel in the same encryption that protects web traffic (HTTPS), so the signalling is confidential and tamper-resistant.
It also authenticates the server to the phone, making it much harder to impersonate your PBX and trick phones into connecting to an attacker.
TLS plus SRTP = an encrypted call
TLS and SRTP are a pair. TLS protects the call's signalling; SRTP protects its audio. Run both and the entire call — setup and conversation — is encrypted end to end across the network. This combination is the baseline for any system handling sensitive conversations.
For browser clients, WebRTC enforces both automatically. For SIP desk phones and softphones, the platform has to offer TLS and SRTP, ideally as the shipped default.
The default test
A simple way to judge a phone system's security posture: is encryption on out of the box, or is it a setting? Systems that generate a signed certificate and enable TLS and SRTP from first boot start secure. Systems that ship in plain text and leave hardening to the administrator usually stay exposed, because the hardening never happens.
Frequently asked questions
What port does SIP over TLS use?
5061 by default, versus 5060 for unencrypted SIP. Secure WebRTC signalling runs over WSS, typically on a separate port such as 7443.
Do I need a certificate for SIP TLS?
Yes — TLS uses a certificate to encrypt and authenticate. Good systems auto-generate a signed certificate at install and let you drop in your own CA certificate to replace it.