How to check if my iPhone is being tracked or tapped
A regular house phone has lots of space to slip in a microphone transmitter! Not really possible in a cellphone, no free space!
But! Your phone could be its own bug! Some Apps are able to monitor your microphone and send the data over the internet without you knowing. Now this is not really possible on an iOS based phone if you've only installed apps from the Apps store as Apple monitors such things. And we know Apple is very paranoid about security!
Some apps which do language translation might need to access to the internet to use the more powerful back end servers to figure out what you've said (they have said) In the case of Siri it to leverages a back end server for some of the work. So unless you've opened the channel it's not possible for these apps to spy on you!
OK, how to prove it! Simple, you'll need to disable cellular services and only have your phone connect to a dedicated WiFi AP. Then have a network sniffer monitor the ethernet traffic from this AP unit (dedicated network to your internet router) While you won't be able to see the encrypted data stream you will be able to see the numbers and size of the data alter as you talk to your phone (within a quite room). If the data stream is consistent when its quite and when you are talking you know its not spying on you ;-}
It's only when you yell out ‘Hey Siri' or tell you phone to translate something to another language will you see additional data.