If you are in an area of poor coverage, be it 2G or 3G you will enter compressed mode. Where you will monitor both 2G and 3G cells, effectively having both transmitters/Receivers active.

There are settings in the Mobile network that control, when compressed mode is entered and how many times with in a period and how many times it is allowed to switch between 2G and 3G within a period. This is as far as I am aware uncontrollable from the Handset.

Yep your correct as it is uncontrolable from the handset.

I can tel you the only time high battery consumption occurs is when registration is incomplete between 2 and 3g signals, under these conditions whilst searching is the high end time, hope this explains.

i found this from ages ago on a pdf in my pc....

Not Registered: Module has to scan and camp, then register on a cell. The radio’s TX/RX have to be
on continue to support the process until the camping completes. No registration or channel is available.
(Power consumption is high during this period).

NB source.... http://www.option.com/Upload/main/Wh...iderations.pdf