Connect Exotel SIP trunking to Smallest AI (Atoms)

11 min

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

Applicability: UI-driven (Atoms “Import SIP” screen) with optional API-driven outbound call triggering.

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).

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.

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.

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.

Engineering detail: smallest/integrations/exotel-vsip/smallest-exotel-voice-ai-connector.md

Quickstart: smallest/integrations/exotel-vsip/QUICKSTART.md

Smallest AI (from official docs)

Topic	Detail
Import SIP	Phone Numbers — Import SIP: Phone Number (E.164), SIP Termination URL, optional Username / Password, SIP Origination URL (provided by Atoms → paste into provider)
Telephony overview	Phone Numbers capability — inbound, outbound, SIP to existing infrastructure
Outbound API	Start an outbound call — 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 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:<trunk_sid> 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)

Phone Numbers → Add Number → Import SIP (guide).

Enter your Exotel DID in E.164.

SIP Termination URL: Exotel edge host/IP and port from Exotel network doc (align transport with Exotel and the Import SIP field).

If using SIP digest with Exotel, set Username / Password to match POST .../credentials on the Exotel trunk.

Save and copy SIP Origination URL for Exotel inbound routing.

Assign the number to your agent (Agent Settings → Phone Number per docs).

Part B — Exotel APIs

Auth: API_KEY:[email protected] · 200 requests/minute (SIP trunk APIs) · _exotel-trunk-api-snippets.md

Outbound SIP (minimal)

Create trunk

Map DID

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):

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": "<PASTE_SIP_ORIGINATION_URI_FROM_ATOMS>" } ] }'

Connect applet (inbound)

Dial whom: sip:<trunk_sid> — Detailed SIP Trunking API ReferenceVoice AI / SIP trunking.

Testing

Outbound: Start an outbound call 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:<trunk_sid>
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