If I called someone can they hack into my iPhone just by being on the phone with me and knowing my phone number? I was also transferred to another person during the phone call, if that would make a difference. Or could they see my WiFi password that's on my phone etc?
While not theoretically impossible, this is highly unlikely.
For this to happend there would need to be a vulnerability in iOS that could be leveraged for this. While there are vulnerabilities in iOS (and in any software), unless I have missed something there is nothing that could be used for an attack just through a phone call.
That does not rule out that there is an unknown vulnerability, a so called zero-day, that makes this possible. By definition we would not know about that. However, I would say that such a vulnerability is quite unlikely, and if one was discovered it would be considered a big deal and be all over the tech news. Apple would be embarassed and forced to quickly publish a patch to fix it.
If such a vulnerability were found, it would probably be used primarily against high value targets (threats to national security, government officials, celebrities) and not against random people. Such a golden egg is valuable, and you would not want to risk it being discovered (and patched) just to hack low value targets.
As an example of a similar vulnerability I should mention Stagefright. By leveraging a bug in a video library it was possible to hack Android phones just by sending an MMS with a malicious video file. The phone would get infected as soon as the MMS was opened.
But to conclude, this is not something I would spend time worrying about when I recieve phone calls.
Well, whilst iOS and others are getting hardened, everyone is ignoring the GSM baseband processors. This second operating system was written years ago, and is proprietary to Qualcomm (and others) and is riddled with bugs and security vulnerabilities.
As IMSI catchers are getting cheaper (e.g. Stingray), it is getting easier to execute malicious code on your device without you even knowing about it.
and a good YT vid from DeepSec 2010 talk 'All your baseband are belong to us': https://www.youtube.com/watch?v=fQqv0v14KKY
The so-called secure phone 'Blackphone 2' is largely pointless.