A CLI route is a wholesale voice path that delivers the calling number you send to the person you are calling. That sounds like the default behaviour of every phone call, and on retail networks it usually is. In wholesale voice it is not guaranteed at all. This guide explains how caller ID actually moves between networks, why so much international traffic arrives without it, and what you should check before trusting a route with your campaigns.
Key takeaways
- A CLI route is a wholesale path where every carrier hop preserves the calling number you send.
- Caller ID is usually lost through least-cost re-routing, header rewriting, bad number formats or regulatory screening.
- Send the same E.164 number in From and P-Asserted-Identity, and only present numbers you hold.
- Verify CLI delivery yourself with real handsets on the destination network, across the day, and repeat weekly.
On this page
CLI, caller ID and the wholesale route
CLI stands for Calling Line Identification. It is the number that the originating network attaches to a call and that the terminating network displays to the called party. In SIP the CLI is carried in the From header and, between carriers, more often in P-Asserted-Identity (PAI). In legacy SS7 signalling it is the Calling Party Number parameter. Whatever the transport, the idea is the same: the number the handset displays should be the number you are authorised to present.
A wholesale route is the sequence of carriers a call takes from your switch to the destination network. Most international calls cross two to four networks. Each of those hops is a commercial agreement, and each one can preserve, rewrite or drop the CLI. A CLI route is a route where every hop has been selected and tested to preserve the calling number end to end. A non-CLI route (sometimes sold as "standard" or "grey") makes no such promise: the call connects, but the handset may show "Unknown", a different number, or a local number the transit carrier inserted.
Why delivered caller ID matters
People answer calls they can identify. Contact-centre data consistently shows a material drop in answer rate when a call shows as withheld or unknown, and an even larger drop when the displayed number is foreign or unfamiliar. For outbound sales, collections and service callbacks, CLI delivery is therefore not a cosmetic feature. It drives the metrics you are paid on.
- Answer rate. A recognisable, local or brand-registered number is answered more often than an anonymous one.
- Callbacks. If the customer misses the call, a valid CLI lets them call you back. Without it the conversation is lost.
- Trust and complaints. Calls from unknown numbers are more likely to be reported. Reports feed the analytics that terminating networks use to label or block traffic.
- Regulation. Many regulators require that the presented CLI is a valid, diallable number the caller is entitled to use. The UK, Germany and the United States all enforce versions of this rule.
How caller ID gets lost
Understanding the failure modes makes it much easier to evaluate a provider. In our testing the same handful of causes explains almost every CLI failure.
Least-cost transit through non-CLI carriers
Wholesale carriers constantly re-route traffic to whichever partner is cheapest that hour. If one of those partners terminates through a channel that does not carry identity, such as a SIM-based gateway or an interconnect that only accepts its own numbers, the CLI is replaced. The route still "works", which is why this failure often goes unnoticed for days.
Header normalisation
Some session border controllers rewrite From and PAI to match their own number ranges, or strip PAI entirely at a trust boundary. A carrier that does not explicitly preserve PAI between trusted peers will lose CLI even on otherwise good paths.
Number format problems
A CLI that is not in clean E.164 format (for example a national format with a leading zero, or a number with a stray + and country code duplicated) is frequently discarded by the terminating network as invalid. Always send + followed by country code and subscriber number, with no spaces or punctuation. Our CLI format checker catches the common mistakes.
Regulatory screening
Terminating networks increasingly screen inbound international calls that present a domestic number. The UK, France and others block or flag international calls showing local mobile CLIs unless the call is known to originate from a roaming customer. This is a feature of the regulation, not a route fault, and a good carrier will tell you in advance which number types will be presented in each market. See our CLI rules by country guide.
CLI route vs non-CLI route
| CLI route | Non-CLI route | |
|---|---|---|
| Caller ID at handset | Your authorised number | Unknown, withheld or a transit number |
| Path selection | Fixed set of tested carriers | Least-cost, changes frequently |
| Typical use | Sales, service, collections, callbacks | Low-value transit, one-way notifications |
| Answer rate | Higher and stable | Lower and variable |
| Monitoring | Per-destination CLI sampling | Connectivity only |
| Compliance posture | CLI ownership checked at onboarding | Often none |
There is a third category worth knowing: CC routes (call-centre or dialer routes). These are CLI routes engineered for the traffic pattern of predictive dialers: high call attempts per second, many short or unanswered calls, and frequent early hang-ups. Not every CLI route tolerates that pattern. Read Short-Duration Traffic and CC Routes before pointing a dialer at a standard CLI route.
How to verify a CLI route yourself
Never take CLI delivery on trust. It takes less than an hour to test properly, and the test should be repeated whenever you see answer rates drift.
Get test handsets on the destination network
Use real SIMs on at least two mobile networks and one fixed line in the country. Test-call services are useful but some use number ranges the terminating carrier treats differently.
Send a clean E.164 CLI you own
Present a number you are authorised to use, formatted as +CC followed by the subscriber number. Record exactly what you sent in From and PAI.
Place calls across the day
Make at least ten calls per handset spread across peak and off-peak hours. Least-cost re-routing often happens at hour boundaries.
Record what was displayed
Note the displayed number, any network label such as "Suspected spam", PDD and audio quality for every call.
Compare with your CDRs
Your call detail records should show the same call with the same CLI. A mismatch points at a rewrite inside your own platform.
Authorisation: the part that keeps routes stable
Delivered caller ID is only valuable if the number is yours to present. Regulators and terminating carriers treat presenting someone else's number as a serious offence, and carriers that tolerate it lose their interconnects. That is why every reputable CLI carrier checks CLI ownership at onboarding. At CLIRoutes this means a short business onboarding, a list of the numbers or ranges you will present, and evidence that you hold them, such as a DID invoice or a letter of authorisation from the number holder.
This is a benefit, not a hurdle. Routes that only carry authorised CLIs keep their good standing with terminating networks, which is exactly what keeps answer rates high month after month. For US and Canadian traffic, ownership also determines the STIR/SHAKEN attestation level your calls receive.
Where caller ID lives in a SIP INVITE
When you troubleshoot CLI, look at the INVITE your platform sends. Three places can carry identity, and carriers do not all read the same one. Send the same E.164 number in From and P-Asserted-Identity, and only add Privacy: id if you intend the number to be withheld from the called party.
INVITE sip:[email protected] SIP/2.0
From: "Northside Support" <sip:[email protected]>;tag=8f2a
To: <sip:[email protected]>
P-Asserted-Identity: <sip:[email protected]>
Contact: <sip:[email protected]:5060>If the From and PAI numbers differ, many carriers present the PAI and ignore From. If PAI is missing, they fall back to From. Mismatches are a frequent cause of "the wrong number is showing" tickets, especially when a PBX inserts an extension number into one header and the trunk number into the other.
CLI delivery and number reputation
Once your CLI reaches the handset, the terminating network and handset analytics decide how to display it. They look at how many calls the number makes, how many are answered, how long they last and how many recipients report or block it. A number that makes thousands of short, unanswered calls a day will be labelled regardless of the route. Spread volume across a reasonable set of numbers you hold, tie each to a brand and campaign, and keep callbacks answered so the number behaves like a real business line.
Choosing a CLI provider
Price per minute is the least useful number on a rate deck if the route does not deliver. Ask how routes are tested, how often CLI is sampled, what happens when a carrier in the path fails, and whether you can see live quality per destination. We collected the questions we would ask in 12 Questions to Ask a Wholesale CLI Provider.
If you are ready to test, CLI routes are available in 100+ countries with instant activation after onboarding. Your rate deck is available in the portal after sign-up.
About the author
Mara Lindqvist, Head of Routing
Mara has spent fifteen years building wholesale voice networks, from TDM interconnects to SIP-only carrier exchanges. She owns route selection, carrier onboarding and quality policy at CLIRoutes.
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