There is no guidance in Prism regarding how or when to call web services. However you could place your services in a module and add it to the catalog.
In this module, when you have finished loading your data asynchronously you could use the
Module Manager to load another module or for example publish an event using the
Event Aggreagator in order to communicate with other modules to perform necessary operations accordingly.
Additionally, you might find this thread handy in order to achieve your requirement:
Best practice when calling services via proxy
I hope you find this information useful.