Automatically Receive Calls App on iPhone using Internet
You cannot do this on an iPhone as Apple does not allow software to take over main-functions of an iPhone.

In the past (and it still probably is) it was possible to add BiteSMS to your iPhone after jailbreaking it. There might be a similar feature for phone calls, but it requires a jailbreak then.

So in short, No, Apple does not support this by default. You might be able to bypass this limit by jailbreaking your device and looking for a tweak which can do this for you.