There is a bit of confusion in the original post.
Your cell phone can not connect to police towers unless the police are spending money I don't know about.
But my cell phone has the ability to connect to towers belonging to a numnber of carriers,, So does yours.. However depending on who your pay your money to it may not be AUTHORIZED for this or that tower... For example it might work on AT&T towers but not T-Mobile (or the other way around) or Century CelluNet towers but not .. Well you get the idea.
Now my phone can work with AT&T, T-mobile and CCN and several others.. But unless my cell company has an AGGREMENT with them, the phone is not authorized to place (or receive) calls on all those towers.
The phoen is compatable with the tower, Just not authorized.
But a 9-1-1 call, by law, the tower HAS to process it, (If it is has the proper codecs for your phone).
That said. Verizon, AT*T/T-Mobile and Sprint/Nextel/Virgin all use different codecs. (Grouped as per the way I typed it)
Any phone that works on a T-Mobile tower can call 9-1-1 on an AT&T tower if that's all there is. Roaming agreement or not (And the other way around).
But Verizon used a differen codec so those towers will not take my calls under any conditions.