Run tcpdump on the PBX and capture all packets from a failing call. Move the capture file to your PC and open it in Wireshark. Look at the incoming RTP, if any.
If there’s none, this seems like a simple networking issue. If it’s present and correctly formatted but silent, I’d open a ticket with Twilio. If it’s somehow ‘bad’, e.g. wrong codec or packet size, post details.
Also, describe your network setup (router/firewall make/model, ISP gateway, etc.)