"Log a Call" is a standard button you'd see on the "Activity History" related list on any enabled Salesforce.com object page layout. Since it's "Log a Call", the subject is naturally set as "Call". We often get asked if we can put in things like "Phone - Outbound", because when it comes to wording certain things, some business people tend to be quite specific. Well, it's a simple request without a straightforward answer. I've seen people overriding the standard button with a VF page that redirects. I'd like to not code if I can, even though coding very much pays my bills. :) My solution is to use a custom button on Task, List Button type, labeled "Log a Call", shown in the same window without bars, and just a plain adapted URL from the standard button with merge fields, such as:
Depending on where it's invoked and what the Subject it needs to have, you can just tweak the parameters and the merge fields. Nice and simple, even an admin can do it.