Is it possible to use the Icon 600 on cloud services to make outbound voice only calls?
The only way to make outbound audio calls from an Icon registered to Cloud, is to register it over SIP to the customer's IPBX, and use that to reach the PSTN network.
Are the SIP settings still available for configuration once the Icon is connected to Lifesize cloud?
Yes, please refer to this post:
Registering a PBX with SIP Registrar 2 on an Icon system paired with LifeSize Cloud
Please read also the comments underneath, you will see that it is a lot simpler after 2.2.4: just use the SIP Registrar 2, no need to enable the menu via the Support tab anymore.
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