Will answer all with my understanding if something will be wrong please point it out.
What i have heard till now that if a UE is supporting relase 10 of 3gpp so it's automatically understood to mind that it will support the previous releases also. Now the question is how it is designed to work.
I think it is up to chip level (mean to say hardware+software). So we can say that the Physical layer software's of GSM/3G/LTE (As per supported bands) will be burned in to the hardware chip which can understand all physical layers of different generations. So that is the reason our phone handware requirements also got increased with growing 3gpp generations.
In more depth: Suppose we have build a physical layer of UMTS that supports WCDMA their protocol functionalities and else everything. Now we have made LTE physical layer which understands their protocol functionalities, OFDMA and else everything. Now a phone should understands both networks UMTS as well as LTE so we will burn both the software's on a hardware chip. That is how i think we can say our phone can support these many frequency bands.
Frequency scan will decide this.
