I do not have a lot of experence with PLB's but have some with ELT's. SARSAT usally comes over about every 40 minutes. With the older ELT's AFRCC would wait for three passes before SAR was called, this will be different for the new ELT's and PLB's. I think the procedure that I read said the the contact number would be called after the first "hit". If you get a PLB make sure it has a GPS built in, I think they all do. Of course like any electronic equipment do not rely on only it because of you do Murphy will make sure it does not work.