I have allready tried to Integrate Circuit using iFrames but that is not allowed by Circuit.
<iframe width="1280" height="749" src="http://circuitsandbox.net/#/"></iframe>
This gives an Error that the website is refusing the connection.
Does anyone know how to implement the same functionality without building the complete app myself?
No, as you pointed out Circuit does not allow to be ran in an iFrame. Circuit is a large application with several panels that would not be ideal in iframes anyways.
There are several examples on how to easily recreate some of the functionality using the JS SDK. See small individual examples at https://circuit.github.io/jssdk.html, or a complete app at https://github.com/circuit/circuit-sdk-app.
Or another good option would be to use the Circuit Web components (https://www.npmjs.com/package/@unify/circuit-web-components) or Vue components (https://www.npmjs.com/package/@unify/vue-components).
Using the Circuit web components for example you can create an app with video conferencing capabilities in just a couple lines of code:
<circuit-call-button
video
domain="circuitsandbox.net"
clientId="f06c51a30f0d4eb6acc05829c3e86266"
target="ce4417ee-d59a-4d37-b486-52e6cef1ac35"
callingText="Starting..."
joinText="Join conference">Start conference</circuit-call-button>
<circuit-call-stage></circuit-call-stage>
Code from https://github.com/circuit/circuit-web-components/blob/master/examples/videoConference.html
or a chat app using the circuit-conversations-list and circuit-chat web component as per https://github.com/circuit/circuit-web-components/blob/master/examples/chat.html