I've done a fair bit of on-call during my career, but I wouldn't do it anymore (at least not for any dollar amount a company would likely pay). I value my time outside work -- and my sleep -- too much.
The standard in SV seems to be that on-call is required and uncompensated (some companies will dishonestly try to suggest that your salary is higher than it would be otherwise). This is gross, and we shouldn't stand for it. There are exceptions of course, but even companies I've heard of that compensate for on-call seem to fall far short of what should be expected and required.
On-call should be compensated at a base per-hour rate (outside of 9am-5pm), with an elevated rate for time spent handling an incident if the pager does go off, with a minimum payout of 1 hour of time. It should be opt-in. If not enough people want to do it to get you the coverage you want, raise the rates until your employees feel that the time is worth the money.
It's pretty messed up that OP here is throwing shade at an employee who is balking at this, wanting to put them on a PIP, plus a thinly-veiled suggestion that they're lying about having asked about on-call requirements during their interview process. Bottom line is that they're drastically changing the terms of employment without offering extra compensation, and while in many jurisdictions that's legal, it's nonetheless unethical and disgusting.