I created a Azure based Web App Bot using the QnAMaker template and used the 'Test in Web Chat' to test it successfully. To enable voice assistant to the chatbot, I configured the Cortana channel and enabled the 'manage used identity through connected service' option too to link the user's account with the skill.
The chatbot wasn't published yet but same Microsoft account has been used in test devices to login to Cortana as the botframework login used to develop the Azure web app bot. However, the invocation phrase doesn't invoke the skill in the Cortana apps running on iOS, Android and Windows 10 PC.
I used utterance such as 'Ask purple genie', 'Tell purple genie' only to get the regular search results.
I changed the language/region to English/US on all the test devices too. Isn't this change enough for testing this skill when the device is not in US.
It turns out to be a question of whether web search or skill has the priority in Cortana skill. In the chat I had with Micromuncher from Microsoft, he worked with his dev team to bring the skill invocation phrase to invoke the skill. Apparently Cortana dev team play around with setting priority of phrases to bring up search results instead of invoking a skill.