The M-series has a comprehensive netmonitor built in called “Eng Field Options”. This menu must be enabled before it can be used. On older phones it was possible with the following keystrokes: [][][]113[]1[] []=pause, hold the * key until [] apperas on the display. If this procedure does not work on your phone you are out of luck. Or at least, if you want to enable this function without the use of the SIM emulator. With the SIM emulator it is possible to set option 113 to 1. See Menu costumization. The eng field options consists of the following submenus:

Active Cell with the most interesting items:

Adjacent Cells

System parameters

Please go to Janus Motpages for an excellent and comprehensive description

With the eng field options it is possible to easily locate GSM basestations. First of all the RxLev option is very useful. This value represents the signal strength of the received signal in dBm. A higher value means a higher signal strength. Please be aware that the value is negative, so -80 is lower than -53!
In practice it does not mean that closer to the BTS does not mean a higher signal strength. In this case also the horizontal and vertical angles are important. If the phone is at right angle to the antenna, vertical as well horizontal (does not apply for omnidirectional antennas), the signal strength will be at its max.

Another useful item is the cell ID. Many providers have the following Cell ID scheme: XXXXY, where XXXX= cell ID and Y=current antenna (1 to 3). Base stations located at straight roads (not at junctions), 2 out of 3 antennas are facing the road (with an angle of course). This means when you are passing this BTS, your phone will switch over from one antenna to the other (not for omnidirectional antennas). This is an indication how close you are to a BTS. Please be aware that the phone has an hysteresis, in practice this is around 400m. It is a pity that signal strength and cell ID cannot be read simultaneously, except if you have two (identical) phones available at the same time.

The third useful item is the Timing Advance. This item is only available at an active call. The TA value (Active cell menu while calling) gives in indication how far you are located from a BTS. 0 means 0 to 547.5m, 1 means 547.5m to 995m etc. Not very accurate, but a nice indication, especially if you are not sure if you are close to the current BTS.