I don’t bother making futile attempts to decide if any service is truly secure, I work with reality. The reality is that you cannot be sure of any of them.
I presume that anything I do using any device or service that can be tied to me, is at risk of third party interception. If I decided to do something that could be embarrassing, or damaging, I would make sure to use both a device and a service that cannot be linked to me in any manner. Not too difficult to do this if you have a background in technology. So Skype, Whatsapp, or any other service is not something I have reason to avoid as there will never be anything of interest to anyone but me and mine. If some fool wants to hack into my conversations with my kids, or Grandkids on any service, let them. If someone wants to track my movement via the portable Broadband WiFi in my car — let them.
I do have a secure phone in my laptop bag — it is off, linked to a provider & data plan that has no association with me or my company, and for added security, is in a Mil-Spec Mu Metal cage. No one can track that unless they are already tracking me when I take it out and turn it on. There are other things you can do for increased security, but few would bother. Things like a very narrow field, hi gain directional antenna. They would need to be between the device and the cell tower.