The recommendation from the FDNY Chief was to give the dispatchers an alert tone - IIRC that was the only radio recommendation that came out of the 1993 bombing - I will try to dig up that report - probably it is a US Fire Admin report
http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201-8.pdf - if you search this document for MAYDAY you will find reference to lost MAYDAYS due to heavy radio traffic - tone alert was mentioned as a cure - (I never thought much of this cure - it was never implemented AFAIK)
http://www.interfire.org/res_file/pdf/Tr-076.pdf - compilation of fire service articles on the 1993 bombing