--- title: Connect Exotel SIP trunking to Smallest AI (Atoms) slug: exotel-agentstream/connect-exotel-sip-trunking-to-smallest-ai-atoms createdAt: 2026-04-01T16:38:40.075Z --- This guide connects **Exotel SIP trunking** to **Smallest AI** **Atoms** using **Import SIP** (bring your own number over SIP). Smallest documents this in the platform **Phone Numbers** flow — you provide a **SIP Termination URL** (where Atoms sends **outbound** SIP toward your carrier) and copy the **SIP Origination URL** that Atoms gives you into your carrier for **inbound** routing. **GitHub repo (reference):** https\://github.com/exotel/AgentStream-VoiceAIEcosystem :::BlockQuote **Applicability:** **UI-driven** (Atoms “Import SIP” screen) with optional **API-driven** outbound call triggering. ::: :::BlockQuote **Exotel edge:** Use **IP\:port** (and transport) from Exotel for SIP toward their gateway (network and firewall). Enter the value Smallest expects as the **termination** target (often host\:port or sip\:host\:port — follow the **Import SIP** form and phone numbers doc). ::: :::BlockQuote **Edge hostnames you may see (India):** in.voip.exotel.com:5070 (TCP) and in.voip.exotel.com:443 (TLS). Use the exact host/IP + port + transport Exotel assigns. See \_exotel-trunk-api-snippets.md. ::: :::BlockQuote **ACL vs digest (important):** Exotel trunk ACL (whitelisted-ips) is intended for **static /32 IPs** only (mask: 32). If a provider publishes only **CIDR/shared egress**, do **not** attempt to whitelist ranges—prefer **digest** and coordinate with Exotel/provider support if calls fail. ::: :::BlockQuote **Origination URL is tenant-specific:** After you add a custom SIP number, Atoms shows a **SIP Origination URL** — **copy it exactly** into Exotel **destination-uris** for PSTN → Exotel → Smallest. Do **not** reuse example hostnames from third-party summaries. ::: :::BlockQuote **Engineering detail:** smallest/integrations/exotel-vsip/smallest-exotel-voice-ai-connector.md ::: :::BlockQuote **Quickstart:** smallest/integrations/exotel-vsip/QUICKSTART.md ::: ## Smallest AI (from official docs) | Topic | Detail | | ------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Import SIP | [Phone Numbers — Import SIP](https://atoms-docs.smallest.ai/platform/deployment/phone-numbers.md): **Phone Number** (E.164), **SIP Termination URL**, optional **Username** / **Password**, **SIP Origination URL** (provided by Atoms → paste into provider) | | [Telephony overview](https://atoms-docs.smallest.ai/intro/capabilities/telephony.md) | [Phone Numbers capability](https://atoms-docs.smallest.ai/intro/capabilities/telephony.md) — inbound, outbound, **SIP** to existing infrastructure | | Outbound API | [Start an outbound call ](https://atoms-docs.smallest.ai/api-reference/calls/start-an-outbound-call.md)— requires agent + linked phone number | | Console | app.smallest.ai | **Codec / security:** Smallest’s public docs describe SIP integration in product terms; confirm **codec** (e.g. G.711 / Opus) and **SRTP** requirements with [Smallest support](https://atoms-docs.smallest.ai/platform/troubleshooting/getting-help.md) if calls fail at SDP negotiation. ## Flows | Direction | What to configure | | ---------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Outbound SIP** (Atoms → Exotel → PSTN) | **Exotel:** create trunk → map DID → **POST .../credentials** (digest). **Smallest:** **Import SIP** — **SIP Termination URL** = Exotel **edge IP\:port** (format per UI); **Username** / **Password** = same as Exotel credentials if you use digest auth. | | **Inbound SIP** (PSTN → Exotel → Atoms) | **Smallest:** copy **SIP Origination URL** from the imported number. **Exotel:** **POST .../destination-uris** on the trunk with that URI (host, port, ;transport= per Atoms). **Flow → Connect:** **sip:\** in **Dial whom**. | **Exotel trunk ACL (whitelisted-ips):** use **only** if Smallest publishes **fixed static SIP egress IPs** you must allow — **one IP per POST**, **mask: 32**. Exotel trunk **does not** support arbitrary CIDR range ACL entries. If Smallest does not publish static egress IPs, rely on **digest** and Smallest’s documented networking. ## Part A — Smallest AI (Atoms) 1. Sign in at app.smallest.ai. 2. **Phone Numbers** → **Add Number** → **Import SIP** ([guide](https://atoms-docs.smallest.ai/platform/deployment/phone-numbers.md)). 3. Enter your Exotel DID in **E.164**. 4. **SIP Termination URL:** Exotel **edge** host/IP and port from [Exotel network doc](https://docs.exotel.com/dynamic-sip-trunking/network-and-firewall-configuration) (align **transport** with Exotel and the Import SIP field). 5. If using SIP digest with Exotel, set **Username** / **Password** to match **POST .../credentials** on the Exotel trunk. 6. Save and copy **SIP Origination URL** for Exotel **inbound** routing. 7. Assign the number to your agent (**Agent Settings → Phone Number** per [docs](https://atoms-docs.smallest.ai/platform/deployment/phone-numbers.md)). ## Part B — Exotel APIs **Auth:** API\_KEY\:API\_TOKEN\@api.in.exotel.com · **200 requests/minute (SIP trunk APIs)** · \_exotel-trunk-api-snippets.md ### Outbound SIP (minimal) 1. Create trunk 2. Map DID 3. POST .../credentials — must match Smallest **Username** / **Password** when used **Optional:** POST .../whitelisted-ips only for each **static** Smallest egress IP Smallest documents (mask: 32). ### Inbound SIP — destination URI on trunk Use the **SIP Origination URL** from Atoms (example shape only — **replace with your copied value**): :::BlockQuote curl -s -X POST "https\://${API\_KEY}:${API\_TOKEN}@${SUBDOMAIN}/v2/accounts/${ACCOUNT\_SID}/trunks/${TRUNK\_SID}/destination-uris" \\ -H "Content-Type: application/json" \\ -d '\{ "destinations": \[ \{ "destination": "\" } ] }' ::: ### Connect applet (inbound) **Dial whom:** **sip:\** — [Detailed SIP Trunking API Reference](docId:2OdDTEDmmf64scwjhGVkT) ## Testing - **Outbound:** [Start an outbound call](https://atoms-docs.smallest.ai/api-reference/calls/start-an-outbound-call.md) with agentId and the imported **phone number**. - **Inbound:** PSTN dial your Exotel DID; call should reach the agent assigned in Atoms. ## Troubleshooting | Symptom | What to check | | --------------------------- | ------------------------------------------------------------------------------------------------------- | | **401** on SIP | Digest mismatch (Exotel /credentials vs Smallest Import SIP) | | **408** / timeout | **Termination URL** wrong **IP\:port** or transport blocked | | Inbound never reaches Atoms | **destination-uris** must match **SIP Origination URL** exactly; **Connect** uses **sip:\** | | Media / codec issues | Confirm codecs and SRTP with Smallest docs or support | ## References | Resource | URL | | ---------------------------------- | ----------------------------------------------------------------------------------------- | | Smallest — app | https\://app.smallest.ai/ | | Atoms — Phone Numbers (Import SIP) | https\://atoms-docs.smallest.ai/platform/deployment/phone-numbers.md | | Atoms — Telephony capability | https\://atoms-docs.smallest.ai/intro/capabilities/telephony.md | | Atoms — Start outbound call API | https\://atoms-docs.smallest.ai/api-reference/calls/start-an-outbound-call.md | | Atoms — Get acquired phone numbers | https\://atoms-docs.smallest.ai/api-reference/phone-numbers/get-acquired-phone-numbers.md | | Exotel SIP API | https\://docs.exotel.com/dynamic-sip-trunking/detailed-sip-trunking-api-reference |