How is what possible?
Calls on K-4 are typically of a sensitive nature. They could have been sent to stand by while the PD SWAT team serves a search warrant. If the POTUS or other VIP comes into town, FD units will be a part of the local support, and they'll be on K-4. All of the FD radios have the encryption key for that talkgroup.
Details of a call can be passed on to a crew much easier via their MDC's in the apparatus, and the crew doesn't have to ask the dispatcher/tactical radio operator to repeat any of it to them, it's right there on the screen, for any call on any channel.
K-5 is not encrypted. I have heard clear transmissions on it.
A special call is simply a request for any other apparatus that may be needed on any particular incident, but that is not part of an upgrade to another alarm level. If a BC only needs one extra engine company or one ladder company at a 3&1 house fire, why ask for an upgrade to a 1st alarm, that would send 2-3 additional engines, an additional ladder company, additional BC's, and other units? Just ask for a special call of one specific type of apparatus that is needed.
John
Peoria