This from Century Link…
CenturyLink High-Speed Internet customers receive full access to all of the lawful content, services, and applications that the Internet has to offer. CenturyLink does not block, prioritize, or degrade any Internet sourced or destined traffic based on application, source, destination, protocol, or port unless it does so in connection with a security practice described in the security policy section below."
Port 25 Port 25 Blocking
CenturyLink filters port 25 to reduce the spread of email viruses and spam (unsolicited email). Email viruses allow malicious software to control infected computers. These viruses direct the infected machines to send email viruses and spam through port 25. Port 25 filtering is a recognized Internet industry best practice for service providers like CenturyLink to filter e-mail traffic. The Messaging Anti-Abuse Working Group (MAAWG), a global organization focused on preserving electronic messaging from online exploits and abuse with the goal of enhancing user trust and confidence, recommends that “providers block incoming traffic to your network from port 25.”
That is the only port they block. as far as I can tell.