Yes. This technique is used by most service providers. I do not know the specifics for your country. It is common for a cell phone to switch frequencies automatically in the background without the user knowing. In fact for any given conversation, your voice is chopped into thousands of small segments that are sent simultaneously on hundreds of different frequencies. This is done partly for security, but mostly for efficient use of the radio spectrum. It can be related to your position, i.e. depending on what cellular base station the signal originates from. Change of position is likely to cause a frequency change as well as change of time. Such frequency changes can happen at any time due to load balancing on the system. All this is transparent to the user especially on modern tri and quad band phones.

NOTE: on most systems even the so called "frequency" such as 850MHZ is really 70 different frequencies between 824 and 894. Multiple channels are needed to handle multiple simultaneous calls from many different users. This is explained in more detail at How Stuff Works.