My former realtor always brings me a big basket of goodies at Christmastime; I would say to add a notification and a consent to that basket. It's true that the implied consent provision in the ECPA would expire after 18 months. It wouldn't carry on for years and years.
I would recommend to the realtors that they build in that consent provision up front. I'd also like to suggest that if organizations had been complying with PIPEDA all along, there wouldn't be a problem today. These rules have been in place for a very long time.