Your answer-seizure ratio to T-Mobile is going down, your customers are hearing fake ringback that never connects, and your CDRs are full of sub-6-second calls that look completed but didn't reach a human. If you're buying numbers and minutes in volume, you've already noticed. The question is whether to swap carriers, rebuild your LCR, or just eat the completion loss.
Short answer: none of those alone. You need a clearer picture of what's actually breaking, and then a carrier mix that doesn't leave you exposed to one upstream's choices.
Why T-Mobile termination is breaking
This isn't one problem — it's three stacked on top of each other.
LRN dips and ported numbers. Most wholesale rate decks bill by NPANXX. That works fine for non-ported numbers, where the block tells you the carrier. The moment a subscriber ports — and T-Mobile has absorbed Sprint plus a decade of porting traffic — the NPANXX lies. Your carrier has to do an LRN dip on every call to figure out where it's actually going. If they don't, or if they cache stale data, calls get routed to the wrong tandem and silently fail.
Feature Group D vs LEC tandem. There are basically two ways into T-Mobile's network. The LEC tandem path goes through the incumbent local exchange carrier's tandem switch and hands off to T-Mobile. The Feature Group D path goes through Sinch, which is T-Mobile's contracted FGD tandem provider, and that path is materially more expensive. Carriers who can't get LEC tandem capacity, or who lose direct interconnects, end up on FGD and the cost gets passed somewhere — usually to you, sometimes hidden in worse-than-expected ASR.
Gray routes in the middle. When a wholesale carrier can't afford FGD and doesn't have LEC tandem, the calls have to go somewhere. That somewhere is often a least-cost route that answers with fake ringback, dumps you into a voicemail box that doesn't deliver, or eats the call entirely. You're paying for completed minutes that never reached a human.
How to spot which path your carrier is using
You don't need to know your carrier's interconnect agreements to figure out if they're routing you well. The CDRs tell you.
Pull 30 days of T-Mobile-terminated calls. Track four metrics per carrier per destination:
- ASR (Answer-Seizure Ratio). What percentage of dial attempts ended in a billable answer. Below 45% to T-Mobile is a red flag.
- ACD (Average Call Duration). Sudden drops mean fake voicemail-answer or one-way audio that hangs up fast.
- PDD (Post-Dial Delay). Anything over 4 seconds suggests a long re-route — your carrier is shopping the call before connecting.
- Short-duration call rate. Percentage of calls under 6 seconds. High numbers mean your calls are answering and dying.
If you're not pulling these already, start this week. Without them you're guessing.
How to structure your LCR
One carrier is a single point of failure. Two carriers is a backup. You want at least four to six in active rotation for outbound, and you want carrier-specific logic — not just least-cost-sorted.
A usable LCR for a bulk operation looks something like this:
1. LRN dip on every outbound call (cache 24-48h max)
2. Branch on terminating OCN/carrier:
- T-Mobile -> Carrier A (LEC tandem) primary,
Carrier B (FGD) secondary,
Carrier C as third try
- AT&T -> rotation across A/B/C/D
- Verizon -> rotation across A/B/C/D
- Other -> rotation, primary first
3. Failover on SIP 408/480/503 or PDD > 6s
4. Track ASR + ACD per carrier per destination weekly
The key move: route by terminating carrier, not just by NPANXX. T-Mobile gets its own decision tree. Everything else can stay on your default rotation because completion to AT&T and Verizon is mostly fine right now.
Should you absorb, switch, or rebuild?
Depends on volume and engineering capacity, not on the headline rate.
Low volume (resellers under ~100k minutes/month): the operational overhead of running multi-carrier LCR isn't worth it. Pick a provider that handles T-Mobile routing well on your behalf, and check their ASR numbers monthly. If they drop, switch.
Mid volume (100k – 2M minutes/month): this is where building your own LCR pays for itself. Add a second and third carrier specifically for T-Mobile traffic. Keep your existing carrier for everything else. You'll claw back most of the completion you're losing within a month.
High volume (2M+ minutes/month): you should already be talking directly to tier-1 carriers about LEC tandem access. The wholesale-of-wholesale layer adds margin you don't need, and you have the volume to skip it.
What about your DID side?
Termination is half the conversation. The other half is inbound: who owns your DIDs, how fast can you port out if your current provider's quality keeps degrading, and what happens to your customers when you switch.
If your bulk DID provider is also your termination carrier and both are getting worse, you have a portability problem on top of a quality problem. The fix is to decouple — buy numbers from one provider, terminate through several. We wrote a bulk DID provider checklist that covers what to look for.
What to do this week
- Pull your last 30 days of CDRs. Filter to calls terminating to T-Mobile OCNs (6529, 6664, plus the absorbed Sprint OCNs). Calculate ASR and short-duration rate.
- If ASR is below 50% or short-duration rate is above 15%, you have a routing quality problem.
- Get conversations going with two or three additional carriers specifically for T-Mobile termination. Ask them which tandem path they use. If they won't tell you, move on.
- Stand up basic LCR with carrier-specific T-Mobile routing. Even a simple primary/secondary/tertiary setup recovers most of the lost completion.
- Decouple your DID supply from your termination if you haven't already. If your numbers and your minutes come from the same upstream, you have no leverage when quality drops.
The T-Mobile situation isn't going back to where it was a few years ago. Build the routing logic now and you'll stop noticing each carrier notice when it lands.