Overview

When an ONT VoIP phone can receive incoming calls but fails to make outgoing calls, the fault almost always lies in outbound SIP signaling, dial-plan mismatch, NAT/firewall traversal, or carrier-side Class 5 softswitch provisioning, not in the physical POTS port or optical layer. This expert FAQ addresses the most common B2B technical support questions from network architects, integrators, and enterprise IT teams deploying VoIP over GPON/EPON ONT platforms.

Frequently Asked Questions

Q1: Why can I receive incoming calls but cannot make outgoing calls on my ONT VoIP phone?

The most common cause is a SIP registration or outbound proxy mismatch, where the ONT registers successfully for inbound INVITE delivery but the carrier softswitch rejects outbound INVITE requests due to incorrect From/To headers, authentication credentials, or dial-plan rules. Verify that the SIP registrar, outbound proxy, and authentication username/password match the carrier-provisioned profile exactly, and confirm the ONT's WAN-side SIP signaling is not being blocked by a stateful firewall or NAT pinhole timeout.

Q2: What is the maximum SIP concurrent call capacity and Gbps throughput supported by carrier-grade ONT VoIP platforms?

Enterprise ONT VoIP platforms typically support 2 to 8 concurrent SIP calls per POTS port with line-rate 1 Gbps or 10 Gbps uplink throughput, depending on the model and codec (G.711/G.729/G.722). This capacity is driven by the embedded DSP resources and the ONT's backplane capacity, which must simultaneously forward data, IPTV, and voice traffic without buffer starvation.

Q3: How do I troubleshoot SIP registration failures that only affect outbound calls on my ONT?

Start by capturing SIP signaling with a packet analyzer on the WAN interface to confirm whether the ONT sends REGISTER and INVITE messages and whether the carrier returns 401, 403, 407, or 503 responses. A 403 Forbidden typically indicates dial-plan or caller-ID restriction, while 407 Proxy Authentication Required points to credential mismatch; also verify that the ONT's SIP timers, transport (UDP/TCP/TLS), and RTP port range align with the softswitch configuration.

Q4: What CLI commands are used to verify and configure VoIP SIP profiles on an ONT?

Typical CLI commands include show voip sip status, show voip sip register, and show voip call-history to inspect registration state and call logs, while configuration uses voip sip profile, voip sip registrar, and voip dial-plan commands. Always save the configuration with write memory or copy running-config startup-config and reboot the ONT to validate persistence.

Q5: How does NAT and firewall traversal affect outbound VoIP calls on ONT devices?

NAT and stateful firewalls break outbound SIP calls because the ONT's private IP and RTP ports are not reachable by the carrier's softswitch unless SIP ALG, STUN, or a session border controller (SBC) is properly configured. Enable SIP ALG on the ONT or route VoIP traffic through an SBC, and ensure the firewall allows outbound UDP/TCP 5060 and the negotiated RTP port range (typically 10000–20000) with a keepalive interval shorter than the NAT binding timeout.

Q6: What dial-plan and digit-map errors cause "cannot make outgoing calls" on ONT VoIP phones?

Dial-plan and digit-map errors are the second most common cause, occurring when the ONT's dial plan does not match the carrier's expected number format, prefix, or emergency-code mapping. Verify the dial plan against the carrier's provisioning template, confirm that the ONT sends the correct number of digits with the right inter-digit timeout, and test with a direct SIP URI call to isolate whether the issue is dial-plan or carrier-side routing.

Q7: How do I test whether the fault is in the ONT or the carrier softswitch?

Connect a known-good SIP phone or softphone with the same credentials directly to the carrier's registrar; if outbound calls work, the fault is in the ONT's VoIP configuration, firmware, or NAT handling. If outbound calls still fail, escalate to the carrier with SIP traces, call-flow diagrams, and timestamps to confirm whether the softswitch is rejecting the INVITE or the ONT is failing to send it.

Q8: What firmware, codec, and interoperability checks should be performed before replacing an ONT VoIP unit?

Before replacing hardware, verify firmware version compatibility with the carrier's softswitch, confirm codec negotiation (G.711u/a, G.729, G.722) matches the carrier profile, and check interoperability with the OLT vendor's OMCI VoIP MIB. Also validate that the replacement ONT supports the same SIP transport, TLS/SRTP requirements, and dial-plan syntax to avoid post-swap outbound call failures.