Five-minute international call check
1. Rebuild the destination
Start with +, the destination calling code and the national significant number. Remove the origin exit code from saved contacts. Apply the destination’s reviewed trunk-prefix exception.
2. Check the origin method
On mobile, + is usually the safest portable representation. On a landline or PBX, confirm the current international access prefix and any outside-line digit.
3. Check account and device state
Confirm signal, balance, international calling permission, roaming status, call barring, carrier-settings updates and any corporate controls.
4. Separate one number from all numbers
If only one destination fails, suspect its number, route or filtering. If all calls fail, start with the device, account and originating network.
5. Capture evidence
Record the exact dialled form, origin network, destination, local time, error message and whether another network worked. Give this to the provider.
Match the symptom to the next check
| Symptom | Possible layer | Action |
|---|---|---|
| “Number invalid” or fast error tone | Wrong exit prefix, duplicated country code, retained trunk prefix, missing digits or unsupported service range. | Rebuild the number from the destination country page and compare every component. |
| Call ends after a delay | No route, unreachable destination, network timeout, congestion or temporary provider failure. | Test another authorised network, record the time and ask the carrier for the release or SIP code. |
| Busy every time | Recipient busy, rejection, call screening, PBX rule or a route translating failure to busy. | Try once later; ask the recipient to check blocking and call logs rather than repeatedly redialling. |
| Works on mobile but not landline | The fixed line may require an exit prefix, block international service or lack a route the mobile carrier supports. | Use the origin’s current international access prefix and confirm service permissions. |
| Works locally but not abroad | Domestic-format number, toll-free geographic limits, roaming restriction or caller-ID screening. | Use the complete international format and request a geographic alternative for toll-free services. |
| One-way audio or silence | VoIP media path, NAT/firewall, codec, Wi-Fi or carrier interconnect problem rather than number syntax. | Test another connection and provide the VoIP provider with timestamps and call IDs. |
1. Rule out a formatting failure
Country code
Verify the destination code, not the caller’s code. Shared codes such as +1 require later digits to identify the destination.
Trunk prefix
A domestic leading 0 is often removed after the country code, but Italy and some other plans have significant-zero exceptions.
Exit code
Do not store 00, 011, 0011 or another origin-specific access prefix inside the destination number.
Length and characters
E.164 permits at most 15 digits in the international number. Put extensions outside that field and remove display punctuation.
2. Check device, account and carrier routing
- Device: turn airplane mode off, confirm cellular service, restart if appropriate, and check call forwarding, blocked contacts and carrier-setting updates.
- Account: confirm balance, international calling, roaming and premium or service-number permissions. Business PBXs can require an outside-line digit or administrator policy.
- Route: a carrier may lack a working route to a specific country, range or operator. A second authorised network test is more diagnostic than repeated calls on the same route.
- Destination: the line may be disconnected, busy, out of coverage, forwarding incorrectly or configured to reject unknown or international caller IDs.
- Service range: toll-free, short-code and premium numbers may have geographic or carrier access limits even when ordinary geographic numbers work.
3. Interpret VoIP and SIP clues
A SIP response is evidence about one signalling attempt, not a universal diagnosis. Providers may map network causes differently. Ask for the complete trace or provider explanation when available.
403 Forbidden
The server understood but refused the request; check authentication, policy and provider permissions.
404 Not Found
The destination is not known at that domain or route; check number normalization and routing.
408 Request Timeout
No timely response; investigate signalling reachability, network delay and upstream routes.
484 Address Incomplete
The address lacks required information; compare the complete international destination.
486 Busy Here
The destination reports busy at that contact; retry later rather than changing a valid number.
503 Service Unavailable
The service is temporarily unable to process the request; honour any retry guidance.
608 Rejected
An intermediary or destination rejected the call and no alternative destination is available through that path.
What to send your carrier or VoIP provider
- Origin country, network and account type—without publishing private account credentials.
- Destination country and a redacted number that preserves its prefix and length.
- Exact local time, timezone and at least one recent attempt identifier.
- Dialled format, including whether + or an exit prefix was used.
- Displayed error, announcement, SIP response and call ID if available.
- Result of a controlled test from another permitted network or to another destination.
Fix the right layer
Use the format guide for number construction, the toll-free guide for restricted service ranges, and the roaming guide when the problem appears only while travelling.
Telecom and troubleshooting sources
- ITU-T E.164: The international public telecommunication numbering planInternational Telecommunication Union. Verified 2026-07-19.
- RFC 3261: SIP response codes and client behaviourRFC Editor. Verified 2026-08-19.
- RFC 8688: A Session Initiation Protocol response code for rejected callsRFC Editor. Verified 2026-08-19.
- If you cannot make or receive calls on your iPhoneApple Support. Verified 2026-08-19.
- Unwanted calls and texts: consumer complaint guidanceFederal Communications Commission. Verified 2026-08-19.
See our editorial and sourcing methodology.