This depends entirely on your phone provider and how the router can be configured to provide SIP while acting as bridge and LAN client. Nothing OPNsense specific. You need to ask your provider about that.
You need to configure the WAN interface for whatever method your ISP used to connect:- DHCP- PPPoE- PPPoE with VLAN- ...Who configured the speedport initially? That person should have this data
You need the configuration of the Speedport to activate the WAN port. Nobody here can tell you how to configure that. Your Speedport knows. Can you log in to the UI and look around for how the Internet uplink is configured?I don't know - I don't have a Speedport and neither your provider. WAN configuration is always provider specific. Sometimes you need a username and password. Sometimes the provider needs the MAC address of the device. Etc. Dozens of different ways to configure.Have you asked your provider or researched independently if using your own router/firewall is supported at all?
Then set your WAN interface on the OPNsense to DHCP. If that does not work, contact your ISP. They are the only ones who can help you.
That's me about to perform nuki tsuke (drawing the sword and dealing the first cut) in the style of Muso Jikiden Eishin-ryu. I train and teach Iaido.