The categories under Settings > General > Usage > Cellular Usage are lumped categories when compared with the breakdown from the cellular service provider. The Call Time should match with the total of the call time categories on the bill, that is, those times within an allotment, unlimited time block, promotional time block, etc. The Cellular Network Data includes those uses such as messages, internet use via cellular service (as opposed to by WiFi which is not counted) which includes email and Safari use, but may not include maps and location services depending on how the information is being exchanged. Notice that this usage category is broken down into received and sent so both are accounted for where the carrier may lump them together...breaking the two out becomes difficult if you have a carrier that does not charge or account for received...you need to figure out where those receptions might affect your data comparison with the carriers billing.
Siri should not affect this unless you are using Siri to use the cellular network to gather information, then that usage is part of the cellular data category.
Bottom-line, it is not a really simple matter to compare the gross usage info on the iPhone with the breakdown of the cell service provider...you need to spend a couple billing cycles very carefully playing with the numbers to figure out where the two add up and match up.
