this is tough with ambulance because many times they are not getting the required signature on file for assignment of benefits, correct? That would be a problem and payments are going to go to the patients. If you don't have a valid assignment the carrier really cannot do anything and it would be up to you to collect these (yes sending to collections would be the course of action I would take) Now, BCBS.. (and there are a few other carriers) will NOT honor assignment of benefits for non contracted providers..period. Nothing you can do, states individually have taken them to court to force them to but it gets knocked out every time. That's their incentive for providers contracting with them.
Now for an OVERALL and LONG TERM solution to your problem is to contract with the carriers you don't for obvious reasons and it will reduce these problems. Another thing you might want to do is look at the claims in which you have a valid assignment and the carriers that paid the patient, and determine if any of them are indeed supposed to be honoring assignment and are not. I would also take a look at your policies and procedures in place when a patient is transported, your forms and such are a vital part of the payment processing order.