I am trying to enable PI SOAP adapter to communicate with a webservice over the internet I am not sure about the firewall ports we need to enable in PI and the available service on the internet side (sync) please share your thoughts.
This is very simple requirement. You have to use SOAP receiver adaper.
You import WSDL from the third party web service and use as target message structure.
Search in wsdl location tag and find the URL and use it in the URL field of the config channel.
Search SCN for SOAP adapter receiver scenario. Regarding firewall port opening, Please check how it is configured now. If the port opening is not exist, then talk to network team to add firewall opening for the port 50000 of PI server to the outside network. If you want to handle https then you have to do firewall opening for the port 50001.
Also refer SAP help link and SCN forums for the same topic.