I see your point here. A principled person might refuse to answer the phone, or call back from the business phone, but that is fairly impractical I would admit.
My only point here was it seems reasonable for the employer to ask for the world. If it's really worth getting spun up, perhaps you could find out if there is a better version of this boiler plate which respects your privacy.
For what it's worth, at my own employer, the phrase is not so open ended, and I suspect it's the same with others:
"Users who use their non-BIGCORP-email (read that as personal) or IM accounts for any BIGCORP business agree to provide BIGCORP with full access to those account, in the event such is necessary to comply with a discovery request or legal mandate"
Using your personal im for work business is a world away from
1 - incidental use of your cellphone for work, even things such as securing work email via 2fa;
2 - using your phone to answer work email via remotely revokable gmail access;
3 - using your personal laptop to answer work email, again via gmail
none of 1-3 mean an employer should get the rights to view my personal email which is also on my phone, or to look inside my phone at all
"My only point here was it seems reasonable for the employer to ask for the world."
No, its not. We accept it, but its not reasonable, it just a CYA attitude than infects a lot of our business and political dealings. An employer should ask for exactly what they want and no more.
We shouldn't even accept it on formal terms. If they need that kind of access during legal proceedings then we might, in good faith, provide that kind of access but it should be provided because we have a good business relationship and not because we signed a contract that gave them the right to our first born.
For what it's worth, at my own employer, the phrase is not so open ended, and I suspect it's the same with others:
"Users who use their non-BIGCORP-email (read that as personal) or IM accounts for any BIGCORP business agree to provide BIGCORP with full access to those account, in the event such is necessary to comply with a discovery request or legal mandate"