You will get PIPed for not signing up for being on call before they raise the on-call bonus.
"we need to talk about your on call volunteering"
"Really? I volunteered for 15 hours of on call"
"Well, 15 is the minimum ok? Now it's up to you if you just want to do the bare minimum, or uh..well look at Brian for example, he volunteered for 37 hours of on call"