You always start a web service call with a Login operation. Login returns a session ID, which is required for all subsequent method calls. A session is valid for 3 minutes, and resets with every call made within the 3 minute period. So if you get a session ID and make a few calls in a row you won't need another login call. But if you wait more than 3 minutes, you'll need to issue Login to get another session ID and use that in a subsequent call.
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services">
<ns:return>F0C6EDD44B270B1F3DD0F1492A2A1585</ns:return> <!--Returns SessionId-->