Routing incoming calls to a SIP URI is an advanced feature.
The SIP URI field allows you to route incoming calls to another SIP provider or service reachable over the internet using a SIP address. If you just want to route incoming calls to your registered SIP phone or PBX, leave this field blank.
The SIP provider or service will publish a scheme for accepting calls by SIP URI. Follow their example to specify the URI on your dashboard at https://simonics.com/gw/. For example, the SIP provider CallCentric states that you can receive calls to your CallCentric account using a SIP URI of accountID@in.callcentric.com.
There are public SIP URIs that you can use for testing. Try entering firstname.lastname@example.org to hear music when you call your Google Voice number.