Lets not even talk about the mess of MF Tandems or almost every carrier barebacking the web by slinging raw unencrypted UDP SIP traffic over the internet...
It is possible to build secure systems in this space, but instead we have almost every major telecom carrier running proprietary unmodifiable platforms from long dead companies or projects (Nortel, Metaswitch,etc) and piles of technical debt that are generally worse than the horribly dated and unpatched equipment that comprises their networks.