Reading an SRV record
_sip._udp.example.com. 3600 IN SRV 10 50 5060 sbc1.example.com.
│ │ │ └ target server
│ │ └ port
│ └ weight (load sharing among equal priority)
└ priority (lower is tried first)Two records with priorities 10 and 20 mean "use the first server; if it fails, use the second." If your phones don't fail over when a provider server goes down, check whether the phones are configured to use DNS SRV at all — many are set to a single hostname or IP.
DNS problems that break phones
- The phone uses a DNS server that's unreachable or slow (often a hard-coded old server after an ISP change).
- Firewall content filters block or rewrite DNS answers.
- The provider changed servers and the phone is still pointed at an IP address.
- Long TTLs after a migration — phones keep the old answer until the TTL expires.
Frequently asked questions
What is a SIP SRV record?
A SIP SRV record is a DNS record, defined by RFC 2782 and used for SIP by RFC 3263, that tells phones which server and port to use for a SIP domain over a given transport, such as _sip._udp or _sip._tcp. It also sets priority and weight for failover and load sharing.
How do I look up SIP SRV records?
Enter _sip._udp. followed by your SIP domain in the lookup above and choose SRV. From a command prompt you can run nslookup -type=SRV _sip._udp.example.com on Windows or dig SRV _sip._udp.example.com on Mac and Linux.
Why won't my VoIP phone register?
Common causes are wrong credentials, a DNS lookup that fails or returns an old server, a firewall blocking SIP (port 5060/5061), SIP ALG interference, a NAT timeout, or the provider's account being disabled. Check the phone's status lights and registration status page first.
What happens if there is no SRV record?
Per RFC 3263, a SIP client that finds no SRV record falls back to the domain's A or AAAA record and the default port: 5060 for UDP and TCP, 5061 for TLS.
Can DNS cause poor call quality?
Not during a call — DNS is only used to find the server. But slow or failing DNS causes delayed registration, calls that won't connect, and failover that doesn't happen when a server goes down.
What are _sip._udp, _sip._tcp and _sips._tcp?
They are SRV names for SIP over UDP, SIP over TCP, and secure SIP over TLS. A provider may publish one, two or all three. Your phone's transport setting decides which one it looks up.
Sources & references
- IETF RFC 2782 — DNS SRV records
- IETF RFC 3263 — Locating SIP Servers (NAPTR/SRV)
- IETF RFC 3261 — SIP: Session Initiation Protocol
Written by Eric · Reviewed November 2026