How do I use Whatsapp call feature on iphone4

1) Can I use Whats App call feature on both an iphone4 or an iphone 4S running ios 7.1 or is it only an option on

iphone 4s?

According to whatsapp website

https://www.whatsapp.com/faq/en/iphone/28041117

I am supposed to be able to use Whatsapp call feature to speak with other

whatsapp users in my contacts.

I have an iphone 4 ( not 4s) running ios 7.1.

This is what I have heard.

a) My phone may need to be jailbroken.

This does not seem to be true as I have a non jailbroken iphone 4s running ios7.1 and

an running ios 8 I can use both to make whatsapp calls.

b) That I can use it by installing whatsapp beta.

- What is whatsapp beta is it developed by the whatsapp company.

- I tried installing it straight onto my iphone 4s using this link. It downloaded but I got the message. Unable to Install. Is there any further guidance for installing whatsapp on an iphone 4s using ios7.1?

c) That it needs to be activated.

And this needs to achieved by someone who already has the Whatsapp call feature, calling my phone. I have tried this but it did not work.

Looking for positive knowledge or solutions. As have exhausted all options and my last straw is to uninstall & re-install.