Can't send regular SMS also can't activate iMessage

Hi guys!

I was recently given a secondhand iPhone 4 and I recently bought credit for my phone. I can receive text messages and I can call people. However I am unable to send text messages.

I am with Telstra and I am on Prepaid. I bought a 5 package using the credit I had for text messaging and that still didn't work.

Also, I am not sure whether this is any relation to why I cant send text messages but my iMessage isn't activating.

(I have looked this up on Google and gone through some of the given answers. However I am wary as to resetting the Network settings as the phone was not originally locked to Telstra.Also I am not using iCloud.)

Any help is appreciated.

Kind Regards

CaptainJazzy