Rudd, in an interview on Sunday with BBC One's Andrew Marr, pointed out that spies used to simply "steam-open envelopes or just listen in on phones," but they can't do that with encrypted messaging apps like WhatsApp, Telegram, or Signal. "You can’t have a situation where warranted information is needed, perhaps to stop attacks like the one last week, and it can’t be accessed," continued Rudd.
"... We do want them [tech firms] to recognise that they have a responsibility to engage with government, to engage with law enforcement agencies when there is a terrorist situation," said Rudd. "We would do it all through the carefully thought through legally covered arrangements, but they cannot get away with saying we are different situation. They are not."
This is an example where an analogy breaks down leading to some poor assumptions. It's easy to think of WhatsApp and other messaging services as just another form of phone call, but that is simply not the case. Everything about a text conversation held over the internet is different than a call placed over the phone network.
With a messaging app, the potential for any malicious actor across the planet to easily intercept an unencrypted conversation is real. This is not true for a phone conversation. The need for end-to-end encryption on a generic phone call is unnecessary because phone calls are simply transmitted, they are never stored anywhere. They are broadcasts between two specific players with a couple of routing servers in between.
Breaking into these conversations is difficult, and requires you to have an active tap on the network, which requires physical access to the network or phones themselves. Wireless phones make this a little easier to circumvent since you could intercept the wireless communication between the phone and the base device, but again, the bad actor would have to be present at the time of the call and within range of the wireless phone.
In this system, it is possible for the telco to provide the government access to specific phone numbers when presented with a valid warrant and to listen to future conversations. Notice that when the warrant is delivered they can't suddenly access past conversations. This is drastically different than what the government is asking for messaging providers to deliver.
Messaging apps are moved through servers which could trivially be asked to store every conversation which passes through them. So no longer is it important to be listening at the time of the conversation, if end-to-end encryption is not offered, the technology would theoretically allow conversations to be snooped on years after they occurred. Additionally, the internet is necessarily connected across the globe. By default, communications on the internet are accessible by devices in any location.
End-to-end encryption is the only way for two people to converse over the internet where they can be assured that not just any person with sufficient time could read that conversation at any point in the future. This is vastly different from a phone tap.
Stop trying to hold tech companies to poor analogies of previous communication providers.