Why a dialer gets 503s when channels are still free

You start a predictive campaign. The trunk still shows free channels. A second later the dialer log fills with SIP 503 and congestion, and the leads burn. The channel counter never got close to its ceiling.

That pattern is a calls-per-second limit. CPS counts how many new call setups the trunk will accept in one second. Concurrent channels count how many calls are already up. A carrier can allow a large channel pack and a small CPS number on the same trunk. The dialer hits the small number first, in the second the campaign starts.

VICIfast described that failure on 24 June 2026: a predictive campaign can spike past a low CPS cap in a single second, with the steady-state channel count still modest. EaseDial, on 14 July 2026 (page updated 2 September 2026), describes the trunk side of the same split: many channels, a low CPS ceiling, and a choke when the dialer ramps.

Two counters on one trunk

CPS is the rate of new attempts. Five fresh INVITEs inside one second is 5 CPS for that second, whether those calls later answer, ring out, or die. Concurrent channels are the calls occupying the trunk at one moment: ringing, connected, or still setting up.

PortaOne’s wholesale page (no date on the page) tells operators to assign a limit for call attempts per second, so one customer’s burst does not flood the network. The channel allowance and the CPS allowance are separate controls. A larger channel pack leaves the setup rate where it was.

A 503 in the first second

Predictive dialers place attempts ahead of free agents. You press start, or you resume a paused campaign, and the dialer fires a pile of INVITEs. After that the rate settles. The average over the hour can look fine, and the first second still overflows.

VICIfast’s note from 24 June 2026 says carriers answer that burst with 503 Service Unavailable and congestion, and a rejected attempt can still burn the lead. EaseDial’s 14 July 2026 article says the termination switch returns 503 or 486 Busy Here and drops the extra attempts instead of queueing them. From the dialer, those attempts look like fast failures. No conversation starts.

Both articles point at a second clue. Failures bunch up at campaign start and then thin out. Channel exhaustion stays bad for as long as the floor is full.

An illustration with round numbers

This is a teaching sketch, not a trace from a Kolmisoft system.

Fifteen agents. The dialer places four attempts per agent in the first second. That is 60 INVITEs. The contract allows 100 concurrent channels and 8 new calls per second. At that second the channel counter is 0. The carrier accepts 8 attempts and rejects 52. The channel graph still looks empty. The dialer has already marked 52 leads as tried.

EaseDial published a similar sketch on 14 July 2026: a 10 CPS burst for 5 seconds can demand 50 channels of concurrency when the steady-state need is lower. Size the trunk for the burst, not for the average across the hour.

What MOR documents

MOR documents these ceilings:

• Up to 100 CPS for the whole MOR system.
• One Asterisk server can support up to 50 CPS. That figure is closer to 20 most of the time.
• The number moves with the architecture.

MOR can also cap a single device and a single provider: “Limit up to (number) calls, during (number) seconds” in Device Limits, and the same control in Advanced Provider settings.

When MOR itself stops the attempt, the hangup cause names the limit:

• 243: provider skipped because of CPS limitation
• 244: device call aborted because of CPS limitation

A 503 in the carrier’s response, with no 243 or 244, is the carrier’s cap. A 243 or 244 is the cap on your MOR.

A constant 150 CPS or more into MOR is dangerous if the system has no proxy. Internal CPS limits do not save the server from that load. The “100 CPS” line is the documented ceiling, not a promise that every MOR server will hold 100. Read the Asterisk line with it: one server is nearer 20.

What M4 documents

M4 is the wholesale switch. It documents up to 10,000 simultaneous calls and up to 1,000 CPS; the real number depends on architecture. You cannot upgrade MOR to M4, because the architecture is different.

M4 caps sit in different places from MOR:

• Per origination point or termination point: “Limit up to X calls, during Y seconds” in Advanced OP/TP settings.
• Per dial-peer and termination-point pair, in the dial-peer assignment window.
• A live CPS monitor exists. Some installations do not have it.

Hangup causes when M4’s own cap trips:

• 304: originator’s CPS limit reached
• 329: terminator’s CPS limit reached
• 342: global incoming system CPS limit reached
• 345: terminator reached CPS limit in a dial peer

Disconnect codes for the same events: 802 for the whole system, 826 origination point, 853 termination point, 855 termination point inside a dial peer.

A dialer whose setup rate sits above one Asterisk server is an M4 conversation. Raising the MOR channel pack leaves the Asterisk CPS limit unchanged.

How to tell the limits apart

Work through the CDR and the dialer log in this order.

1. Check the clock. Rejects in the first seconds after start or resume point at CPS first.
2. Check the channel counter at that same second. Free channels plus 503 means the setup rate, not the channel pack.
3. Check who sent the reject. Carrier 503 or congestion, and no Kolmisoft CPS hangup, points at the carrier’s cap. Hangup 243 or 244 on MOR, or 304, 329, 342, or 345 on M4, points at a cap you configured or at the platform ceiling.
4. Check the rest of the hour. A floor that stays failed while channels are pegged is a concurrency problem. A floor that fails in spikes and then runs is a CPS problem.

VICIfast also notes that hammering the carrier past its CPS cap can get the trunk limited harder or suspended. Slow the dialer. Retrying the 503s faster feeds the same cap.

What to ask the carrier

Ask for two numbers in the service agreement: concurrent channels, and new call attempts per second. EaseDial’s 14 July 2026 piece says a quote that lists channels and skips CPS is incomplete for dialer traffic, and a missing CPS figure is a signal in itself.

If one trunk’s CPS is the bottleneck and the switch can hold more, VICIfast’s 24 June 2026 article says spreading the load across more than one provider buys headroom. Each provider keeps its own cap. Add the caps. They do not stack inside one trunk.

On MOR, set the device or provider limit at or under the carrier’s CPS number. MOR then stops the extra attempts with 243 or 244, so those attempts do not go out to that provider. If the dialer must place more setups than one Asterisk server can start, M4 documents 1,000 CPS and 10,000 simultaneous calls. There is no control on MOR that turns it into that switch.

Keep reading

A slow ring after the INVITE is a different clock. Post-dial delay is the gap from INVITE to the first 180, 183, or a negative response. A high setup rate makes that gap worse, because the switch is busy setting up calls. A long PDD with no 503 is a delay problem, not a CPS cap.

If the next constraint is the hours a dialer may run, the neighbor article is time-based routing for dialer calls. That post is about when the route is open. This one is about how fast setups may start while the route is open.

FAQ

Is CPS the same number as concurrent channels?

CPS is new setups per second. Concurrent channels are calls up at one moment. You can sit under the channel ceiling and still receive 503s.

What CPS does MOR document?

Up to 100 CPS for the whole MOR system. One Asterisk server can reach 50, and operators should expect closer to 20 most of the time. Device and provider limits can sit lower.

What CPS does M4 document?

Up to 1,000 CPS for the whole system, and up to 10,000 simultaneous calls. Both depend on architecture. Origination points, termination points, and dial peers can carry their own limits. M4 is a separate product from MOR.

Which hangup means the switch stopped the call?

On MOR, 243 and 244. On M4, 304, 329, 342, and 345, with disconnect codes 802, 826, 853, and 855. A carrier 503 with none of those codes is the other side.