Hi Katherine,
It's a little bit more legitimate for a client to want to get in touch via calling your phone than via Hangouts — some of them will need to get in touch with you quickly and without warning, and calling is the best way to do that.
That said, a client's insisting on texting over using the Upwork app is a little shady, because there's nothing you can do via texting that you can't do via the app. Since you seem to be a new user, and are thus a likely target for scammers, you're right to be suspicious.
Personally, I never communicate with a new client outside the Upwork app. There's really no need to. If you're at all suspicious, politely tell the client that you prefer to use the app. No reasonable client will have a problem with that. If they insist on texting ... they're probably not legit.
Hope this helps!
