The collection agencies will eventually start calling you, but if you notify them in writing that they cannot call, they must stop, or you can sue them for up to $1000. This is because of the Fair Debt Collection Practices Act.
After about 6 months to one year or longer, some collection agencies go to court to get judgments, and some collection agencies sell the debts to other collection agencies, which will start to call you, and then if you notify them, they must stop, etc.
If any collection agency gets a judgment, that judgment can be used to garnish wages (if you take home more then $217.50 per week), and bank accounts, and can be used to place liens on property.
It is unlikely that any judgment will be gotten before one year, as the court process takes at least a few months (you have to be served the papers, and have to go to court, where you are allowed at least one postponement, which will be for at least another few months).