Yes, many explosion-proof SIP telephones support NTP/SNTP time sync, and some can learn NTP servers from DHCP. Accurate time improves logs, SIP records, and event correlation across PAGA, VMS, and SCADA.

Refinery worker on emergency call beside smoke, wall-mounted industrial phone on pipes — Refinery Emergency Call

What “time sync support” should mean for an Ex telephone

Time is a safety and maintenance feature, not a cosmetic setting

In hazardous plants, an Ex phone is often used in emergency workflows. That means NTP time synchronization ¹ accuracy affects more than call history. It affects how teams prove response time, how they trace who pressed SOS, and how they link a phone event to an alarm on another system. A device that drifts 3–10 minutes can still “work,” but it breaks trust in the records.

Define time performance in the same way you define IP/IK

Many tenders specify IP66/67 and IK10, but they forget time. The result is a phone that has a clock, but it is not managed. A better approach is to define:

time protocol support (NTPv4/SNTP)
server redundancy and failover
security hardening (auth, ACLs, NTS if required)
logging format and timestamp sources
behavior when the network is down

A practical acceptance checklist that keeps procurement simple

The easiest way to avoid debate is to request a short, measurable test.

Item	Target requirement	Field test idea
Sync method	NTP/SNTP + DHCP option support	set NTP, reboot, confirm time correct
Accuracy	stable after 24–72 hours	compare to a trusted time source
Redundancy	2–4 NTP servers	shut one server, confirm failover
Security	limit who can query/serve	scan and verify ACL/firewall rules
Logging	consistent UTC or local time policy	compare phone logs to syslog timestamps

A simple note from real deployments: most “time problems” are not bugs in the phone. They come from blocked UDP 123, bad DNS, or a site policy that only allows local time servers.

Which time protocols are supported—NTPv4/SNTP, DHCP option 42, or SIP Date header fallback?

Most industrial SIP endpoints support SNTP or full NTP client mode, many can accept NTP servers via DHCP option 42, and some can fall back to SIP Date header time during registration. The right choice depends on network policy and security needs.

SIP provisioning diagram with server, app, and cloud connectivity icons — SIP Provisioning Diagram

NTP vs SNTP in practical terms

Many vendors use SNTP ² in UI wording even when the client behaves like a standard NTP client. For most Ex telephones, the difference that matters is not the label. It is:

can the phone poll multiple servers?
does it keep time stable through reboots and link flaps?
does it expose sync status and last update time?

DHCP option 42: fast rollouts and less manual work

DHCP option 42 ³ can deliver NTP server addresses. This is useful when many phones are deployed across units and tunnels. It allows “plug in and sync” without opening a web UI. It also fits sites where endpoint config is locked and only DHCP is allowed to set core network parameters.

A common trap is mixed DHCP scopes. One plant VLAN might hand out NTP servers. Another might not. Then phones on one unit have correct time and phones on another do not. The fix is to treat NTP server assignment as a standard part of the DHCP template.

Can multiple NTP servers be configured with authentication, stratum preference, and timezone/DST rules?

Yes, many industrial SIP phones allow multiple NTP servers, and better designs support server priority and clear failover behavior. Authentication support varies by vendor, while timezone and DST rules are usually supported as local display settings rather than affecting the actual UTC timekeeping.

Primary and secondary server racks for IP phone NTP/HTTPS redundancy and failover — Server Redundancy Setup

Stratum: do not chase a number, chase a stable hierarchy

Stratum ⁴ is a useful concept, but a phone rarely needs to “choose by stratum” if the network design is correct. The better approach is:

set the plant’s authoritative local time servers
keep them disciplined by a GPS source or upstream reliable source
point phones only to those local servers

How to harden NTP—ACLs, firewalls, NTS, and offline local time servers in industrial networks?

Harden NTP by limiting who can talk to time servers, using ACLs and firewalls to control UDP 123, and preferring local offline time servers in plants. NTS can add cryptographic protection, but many sites achieve strong security through segmentation and strict access rules.

Internal NTP voice VLAN diagram with firewall ACL UDP 123 permitting or blocking — NTP Voice VLAN

NTS: when it matters and when it is too heavy

Network Time Security ⁵ (NTS) can protect NTP exchanges with modern cryptography. It is valuable in networks where time integrity is critical for compliance or where attackers could access segments to spoof time.

Does accurate time improve logs, syslog, SIP timestamps, and event correlation with PAGA, VMS, and SCADA?

Accurate time makes logs trustworthy: phone call records, syslog entries, SIP signaling timestamps, and alarm events align across PAGA, VMS, and SCADA. This improves incident response, compliance reporting, and maintenance efficiency.

Real-time VoIP monitoring dashboard with call analytics and mobile control app interface — VoIP Monitoring Dashboard

Syslog and SIEM value depends on time

Many plants forward device logs to syslog or a central SIEM ⁶ monitoring platform. Time is the glue that lets a SIEM correlate events from switches, routers, SIP servers, and phones.

SIP records and call quality investigations

Accurate time helps call detail records ⁷ (CDR) and answered time tracking. In plant networks, this can separate “PBX issue” from “switch policy issue” in minutes by aligning quality metrics with network congestion windows.

Conclusion

Yes, Ex SIP phones can sync time with NTP/SNTP. A clear multi-server and hardening plan turns timestamps into evidence, not guesses. For support: info@sipintercommanufacturer.com

Footnotes

Learn how the Network Time Protocol ensures synchronized clocks across computer systems over packet-switched networks. ↩ ↩
A simplified version of NTP designed for devices that do not require the full complexity of the protocol. ↩ ↩
Technical specification for the DHCP option used to assign Network Time Protocol servers to network clients. ↩ ↩
Understanding the hierarchical levels of time sources in a network to determine the distance from the reference clock. ↩ ↩
An extension to NTP that provides cryptographic security for time synchronization to prevent packet manipulation. ↩ ↩
Software that provides real-time analysis of security alerts generated by applications and network hardware. ↩ ↩
Detailed data records produced by telephone exchanges containing information about individual telecommunication transactions. ↩ ↩

About The Author

DJSLink R&D Team

DJSLink China's top SIP Audio And Video Communication Solutions manufacturer & factory .
Over the past 15 years, we have not only provided reliable, secure, clear, high-quality audio and video products and services, but we also take care of the delivery of your projects, ensuring your success in the local market and helping you to build a strong reputation.