re: "Is there a way to contact a freelancer when they are offline? I need clarification of an issue pretty urgently in order to meet a dealine."
Upwork has very clear rules about this.
You are welcome to contact freelancers and clients using any method that is mutually agreed upon. You can call people, Skype them, use Facebook, send them email, send them a letter. You can even go to their house.
But only if the other person has consented to you doing so.
If an Upwork user has provided you with contact info or methods OUTSIDE of Upwork, you MAY USE IT. If they didn't provide you with that contact info, you may NOT use it.
If somebody provides you with an email address BUT DID NOT provide you with their phone number... then you may email them using that email address. You MAY NOT call them, even if you figure out their phone number.
Here are simple examples:
Question: I am working with Fred on a project via Upwork. Fred gave me his email address. Is it okay for me to send an email message to Fred if I really need to talk to him about something, and he isn't online?
Answer: Yes. This is fine.
Question: I am working with Bill. I know his name. I did some google searches and digging around on some websites, and I found a phone number for Bill. Is it okay if I call him using this phone number?
Answer: No. This is serious violation of Upwork ToS. If you do this, you could have your Upwork account terminated.
re: "Is there a way to contact a freelancer when they are offline?"
Here is an additional hint, although it is not directly related to your question:
Don't check if people are "online."
If you want to send somebody a message, then send them a message. They will reply when they reply.
You don't need to know if somebody is "online." And you have no way of knowing if they are actually "online" or not.
What if there is some little dot (let's call it a "stalker dot") that turns a certain color or something when somebody is on Upwork. Can you rely on that? Not really. What if they set their Upwork app to indicate they are NOT online, even though they are really online? What then? You think they are offline, but they are really online. And what if you see the indicator that they are online? But they really just left their computer on and their app running, but they went out to eat. So you're frantically messaging them and wondering why they don't reply. Upwork isn't Facebook or some other social media system.