sforce.opencti.saveLog() and sforce.opencti.screenPop() methods can be used to create and open Case record in Salesforce using Open CTI.

Sample Code:

sforce.opencti.saveLog( {
    value: { 
		entityApiName : "Case", 
		Subject : "Testing", 
		Priority : "Low",
		Origin : "Phone"
	},
    callback: function( result ) {
        console.log( 
			'Result is', 
			JSON.stringify( result ) 
		);
        console.log( 
			'Record Id is', 
			result.returnValue.recordId 
		);
        sforce.opencti.screenPop( {
            type : sforce.opencti.SCREENPOP_TYPE.SOBJECT,
            params : { 
				recordId : result.returnValue.recordId 
			},
            callback : function( result ) { 
				console.log( 
					'Result is',
					JSON.stringify( result ) 
				) 
			}
        } );
    }
} );