Solution Approaches

ServiceNow provides 2 mechanisms for the integration of the CTI Toolbar. 

1- SNow UI Pages 
As you may know, in ServiceNow, we can create a Custom UI page. With the help of those pages, we can then create a server script and client script. In this approach, SNow supports integrating the CTI Toolbar via iframe. Attached is one snippet of the iframe. This is the iframe you are referring to. 

2- OpenFrame Configuration 
There is another approach to integrating the CTI Toolbar directly in the open-frame Configuration rather than creating the UI Page -> Iframe.  Attach is the screenshot of the open-frame Configuration page. In this approach, there is no such iframe link. 

In the above-highlighted part, we can provide the CTI URL(We will deploy our connector in your premise and obtain the URL). 

Our connector supports both models, please let us know which model you prefer for your customer, we can go with any of the above. 

INFO

Out-Of-the-Box, our connector support 1st approach, this is the approach recommended by ServiceNow as well