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US carriers judge outbound traffic on behavior: campaigns with too many abandoned or very short calls get flagged as “Spam Likely”, lose answer rates, and incur Plivo surcharges. Staying deliverable takes two things covered here, keeping your traffic within Plivo’s quality thresholds and pacing your calls within your allocated CPS (calls per second), which on the Voice API queues calls above the limit and dials them once your rate drops. This guide applies to US destinations only; expectations, limits, and carrier behavior differ in other countries.
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Why call behavior matters

US carriers continuously analyze call traffic for patterns associated with unwanted calling. Two signals matter most for outbound voice agent campaigns:
  • Answer rate: the share of your call attempts that recipients pick up
  • Call duration: how long answered calls last
Campaigns with low answer rates and a high share of very short calls look like untargeted cold calling to carriers and analytics providers. The consequences compound: your numbers get flagged as “Spam Likely” on recipients’ devices, answer rates drop further, and your traffic may be subject to surcharges or account review. Calling only recipients who expect your calls, with proper consent where required, is the single most effective way to keep these metrics healthy.

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Call quality thresholds

Plivo enforces monthly thresholds on two categories of outbound calls: Calls above these thresholds incur per-call surcharges. See Abandoned and short-duration call thresholds for current surcharge rates. These thresholds apply to both SIP trunking and audio streaming traffic, and exclude calls to India. Rates moderately above the thresholds indicate low-intent traffic: for example, stale contact lists or overly aggressive campaign pacing. Substantially higher abandonment rates are a strong signal of untargeted cold calling and can trigger an account review.

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Keep your metrics healthy

Voice agent campaigns commonly trip these thresholds through automation choices that are easy to fix:
  • Call opted-in, expecting recipients. Purchased or scraped lists produce low answer rates by definition.
  • Pace campaigns within your CPS limit. Configure your platform’s batch or campaign dialer conservatively; launching more call attempts than recipients can answer creates abandoned calls.
  • Do not hang up on every voicemail. An agent that detects an answering machine and immediately disconnects generates short-duration calls at scale. Leave a brief message or schedule a retry instead. On audio streaming and Voice API calls, use machine detection; on SIP trunking, use your orchestration platform’s voicemail detection.
  • Make the opening line count. Recipients who hang up within the first few seconds create short-duration calls. Have your agent identify itself and the reason for the call immediately.
  • Establish your caller identity. Use a Plivo number you own as caller ID for full STIR/SHAKEN attestation, and register your business name with Caller Reputation so recipients see who is calling.
  • Spread retries out. Redialing the same number many times in a short window increases abandonment and spam complaints. Keep contact lists current and remove numbers that repeatedly go unanswered.

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How CPS allocation works

CPS (calls per second) controls how fast you can initiate new outbound calls with the Make a Call API. All accounts start with a default of 2 CPS. See Account Limits for how CPS relates to concurrent calls.
Calls above your CPS limit are queued, not rejected. Plivo accepts outbound Call API requests beyond your CPS limit and dequeues them as your calling rate drops below it. Queued calls dial later than requested, so if your campaign timing matters (for example, calls tied to appointment windows), pace your API requests within your CPS limit rather than relying on the queue. (Calls placed through SIP trunking behave differently: attempts above the CPS limit are rejected.)

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Estimate the CPS you actually need

CPS requirements are usually much lower than teams expect, because CPS governs call initiation rate, not simultaneous call capacity: As a rule of thumb, your sustained CPS need is roughly your peak concurrent calls divided by your average call duration in seconds. For example, a fleet of agents holding 100 simultaneous conversations that average 3 minutes needs less than 1 CPS sustained. Even very large dialing operations placing tens of millions of calls per month typically operate below 60 CPS.

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Start small and ramp up

CPS increases are granted incrementally, based on demonstrated traffic quality:
  1. Start at the default (2 CPS), or up to 5 CPS for initial evaluation. This is sufficient for pilots and proof-of-concept traffic.
  2. Run live traffic for 2 to 3 weeks. This produces the call statistics (answer rate, abandoned-call rate, short-duration-call rate) used to assess your traffic.
  3. Request an increase with your traffic history. Contact Plivo support or sales. Higher CPS allocations require a minimum usage commitment and traffic that stays within the quality thresholds above.
Large CPS allocations are not granted upfront to new accounts, regardless of anticipated volume. This protects call quality for all customers on the platform, and for your own traffic: ramping up gradually is also how carriers build trust in your numbers.