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You can also add DLs to extensions and call to DLs logging in to 2 different devices to make test calls. Here is how you can add DLs : https://support.ringcentral.com/article/ringcentral-phone-softphone-outbound-calling-adding-digital-line.html

Login your sandbox service web and add a new user with device (at the select a device step, choose others and select the RingCentral soft phone as the device).

Now you can make test calls between users in your sandbox account (something like 101 and 102 users) using the soft phone (switch to sandbox mode before logging in).