If you use Salesforce, you could work with your SFDC admin to create an activity type of "inbound call" that gets logged on lead activity history. This is probably also contingent on you having some sort of call tracking platform that is integrated with the CRM (something like a NewVoiceMedia, Connect&Sell or PowerDialer).
Assuming you have that in place, you can then set up a trigger in Marketo that listens for Activity is Logged in the smart list and set the modifier to "is" or "contains" the name of the activity type that was created (i.e. "Inbound Call"). The flow could then be to add those leads to a list, send out an alert, etc.
+1 to Nate! Triggering off activites works well. You do need a well-defined scheme around your activity fields and also rules in place to ensure data is recorded in a consistent and predictable way, so that your triggers always work.
Justin Norris | Perkuto
To add to this thread - what if you don't have SalesForce? We use DialogTech for call tracking, don't have Salesforce and don't have an integration with our CRM, so to get them in Marketo I need to import a list. BUT I want to make sure all of the acquisition data, lead created date, etc. reflects they date that they called in with an interesting moment for that day not the day I upload the list. Is that possible? What other factors do I need to consider for a well-build program that shows ROI for call ins?