I understand that 4G devices don't always use the 4G network for voice, they may either use 2 radios, or drop the 4G connection. The 4G network is used for data, and a 2G network for voice/SMS.
Eventually they will all start sending voice over the data link, but it isn't guaranteed.
As far as I know, none of the UK macro operators support Voice Over LTE yet, so yes, if you receive a phone call on 4G you get a paging message telling you to fall back to 2/3G circuit switched and answer the damn thing.
I'm assuming this is my stock S5 won't let me choose 4G only (calls become impossible), where as I can lock it to _just_ 3G.
I don't feel this fully addresses the original statement of "no 4G, hacked", though. If the downgrade-to-2G attack is the worry then 3G & 4G should be perfectly fine.
