If your Lifesize Icon is registered to any SIP server for VOICE calls under SIP2 section of Icon, Now you can make VOICE only call via SIP2 Directly. It will not Go to SIP1 and fall back to SIP2.
Please find the attached document for the Steps and details.
1. Upgrade Icon to 3.7.1 version. (Contact Lifesize Support so we will push LS_RM3_3.7.1 (3191) version to Icon)
Please share Serial number of the device, we will push from server. Its not Available in GA since its Hotfix as of now(October 2018).
2. Login to Icon web UI with Username - support, Password - support.
Under Support -> SIP PBX Secondary -> Default on Voice Calls -> Check this check box.
3. Select "Voice" in the dial pad of the Lifesize Phone while making Call.
4. Then calls should route via SIP2 ONLY (no retries over Lifesize Cloud)
Please note: If VIDEO is selected, Call wil always first try with Lifesize cloud.