# Three ways a call can enter · Learn it in 5 minutes

- The technical requirements describe three SIP stories. Five9 can provide ITSP trunks. Your ITSP can connect to Five9 session border controllers, and there is a checklist for that. Or a gateway you already own — Avaya, Cisco, or Asterisk — can talk to Five9 over the internet, a VPN, or Direct Connect.
- Cisco in that sentence is a gateway type, not a promise that a UCCX script will import. Pick the story before you draw a codec or a DNIS.

## Sources and go deeper

- [Technical requirements PDF](https://documentation-be.five9.com/bundle/five9-technical-requirements/raw/resource/enus/five9-technical-requirements.pdf). vendor. last checked 2026-10. Fetched for this lab in October 2026. Re-read before you buy hardware or open a firewall.
- [SIP trunk options chapter](https://documentation.five9.com/bundle/technical-reqs/page/technical-reqs/SIP-trunk-options/_ch-five9-sip-trunk-options.htm). vendor. last checked 2026-10.
- [RFC 3261](https://www.rfc-editor.org/rfc/rfc3261). rfc. last checked 2026-10.
- [RFC 3550, RTP](https://www.rfc-editor.org/rfc/rfc3550). rfc. last checked 2026-10.
- [RFC 3551, RTP profile for audio and video](https://www.rfc-editor.org/rfc/rfc3551). rfc. last checked 2026-10.
- [ITU-T G.711](https://www.itu.int/rec/T-REC-G.711). standard. last checked 2026-10.
- [ITU-T E.164, international public telecommunication numbering](https://www.itu.int/rec/T-REC-E.164). standard. last checked 2026-10.