I'd like to add my two cents... The best reason for not having phone
patches on trunked systems is that a phone call usually lasts far longer
than a radio transmission.  There are a finite number of frequencies in
the particular public safety system, and the inordinate length of a
phone call takes 1/x of the system away from general use.
I am factory trained on Ericsson trunked systems, and had a major hand
in one for a year.  The decision was not lightly made to not allow phone
patches.  
Generally, a transmitter site will have 5-8 frequencies, with some major
cities running up to 25 or so frequencies.  Let us say that the
particular site running the phone patch has 5 frequencies - suitable for
a smaller community or area.  One frequency is designated as the control
channel, and continuously transmits.  We are left with 4 working
frequencies.  Allow a phone patch on one, and you have cut the available
working frequencies by 25 percent.  With good trunked systems running in
the millions (yes, I said millions!) of dollars, you can see why eating
the cellular bill is understandable.
Duane
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