Having designed a buck regulator for product incorporating a Wavecom GSM module to use with a 12V battery 2 years ago, I know the issues you may be facing: a) certain switching frequencies can be forbidden as they might 'modulate' the RF output violating the PTCRB pre-approvals of the module, b) digital GSM RF transmissions is done as short pulses ever few milliseconds, so the buck regulator needs to react very quickly without overshooting and creating issues with the module, c) the large difference between the RX current and TX maximum current means you need a buck regulator that's efficient at both low and high currents, and, if operating off a battery only once or twice a day, you need one that has very low shutdown current.
What I used successfully was a Linear Tech LT3481, and programmed it for about 1.1 MHz. Had to be careful selecting the inductor as many high-current SMT inductors are not designed for the frequency (mainly a skin-effect and wire-size issue).
Between National, Maxim, TI, Linear Tech, Micrel, and others, there are lots of choices in switching regulators which can get you confused as to which one is the "BEST" choice. What I used may not be the best for your application, but may provide another one for you to look at.