The server task will remain there, waiting for clients to connect, until you exit it - red X top right.
What iPhone app? Assuming its a browser, enter "http://<server name or ip address>:2500" as the URL, if that works bookmark it as Calibre Server. If it doesn't work, ping the server - if that doesn't respond then your iPhone can't see the server - you need to resolve that first.
BR
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