Unfortunately, everything about Google Voice/Hangouts/Talk/Whatever-it-is now sucks. Trying to find someone is damn near impossible. I totally can attest to that. The removal of federated and regular XMPP was the wrong direction.
I'm in the wonderfully weird situation of not having a Gmail account either. I deleted it and ran my own mail server back in 2012. I use DavDroid/Radicle for contacts, so in the web interface, any contacts after 2012 that are SMS are phone number only. I have to use my phone to see the names.
I don't think it's even possible to find someone via gmail address anymore. Occasionally I'd find someone on Google Plus .. and have no freakin idea how the hell to send them a personal message (either in G+ or Hangouts).
The whole Hangouts/G+/Gmail ecosystem is awful. If you don't have a gmail account, it's beyond unusable. I pretty much just use Hangouts for legacy chats.
AIM/Yahoo/MSN all worked .. and now they're all gone (I think Yahoo is still there, but web only). Facebook was unreliable as shit and god awful until around 2013/2014. It took them that long to create someone their competitors had done better a decade ago.