Is it safe to call if you're undocumented?
Absolutely! Nobody will ever ask if you're legal or not or put your name or number on any kind of immigration list. Congressional offices have absolutely nothing to do with ICE or any kind of immigration enforcement. It is absolutely 100% safe to call, it's probably safer than starting a new email account.
We've been calling for months, some for years and we're still here. People on this forum have written letters in their own name, still here. Some of them met Senators in person and everyone knew they were undocumented and they are all still here. Nobody there has any reason to report you, they never do that. We're all a little afraid at the beginning but there is really no reason to.