If you are trying to avoid police, or if something happens to you and you fear being traced by such means (which, is against the constitution, I believe, since that would be considered entrapment...) turn your cell phone off, and remove the battery, and SIM chip if you can.
Or just completely destroy your cell phone...
Other than that, police can not "tap" into your phone, nor can anyone else, unless you are in Analog mode, and voice privacy is unavailable.
Your phone will usually notify you if privacy isnt available.
It might also be useful for you to know:
They can only find out what cell phone tower(s) you are connected to... which, is only an approximate location... in some cases, its about as good as using an IP address to find where someone is.