OptimizeVoip link looks to be outdated, not sure. Even for Voip, now no need of separate Voip certificate. Single generic certificate with Voip topic should suffize. Also, using JWT, Voip should be supported. (As per apple's Doc for JWT below:)
The HTTP/2-based provider connection is valid for delivery to one specific app, identified by the topic (the app bundle ID) specified in the certificate. Depending on how you configure and provision your APNs Transport Layer Security (TLS) certificate, the trusted connection can also be valid for delivery of remote notifications to other items associated with your app, including Apple Watch complications and voice-over-Internet Protocol (VoIP) services. APNs delivers these notifications even when those items are running in the background. See APNs Provider API for details, and see Voice Over IP (VoIP) Best Practices in Energy Efficiency Guide for iOS Apps.