How Do You Call Back a SIP Phone, or Is It Even Possible?

The simple answer is:
Generally speaking, no, PSTN landline phones cannot directly call SIP numbers directly.

SIP numbers (URI's) can only call each other over the Internet. A URI is just like an Email address. Just as you can send email from one email account to another email account, so too can VoIP calls be made from one VoIP user to another. But, in this case, VoIP calls can only be made between different VoIP providers when they have "Peering Agreements" between their VoIP networks. Otherwise, VoIP will not traverse between private networks without the peering agreements.

When you call a PSTN landline phone (or cell), what they see on their caller ID can be most anything and not necessarily your VoIP virtual number. By the way, Virtual Number is a rather loose term. It can mean anything from your VoIP account number, your URI number, or your DID (Direct Inbound Dialing) phone number. DID is the more correct industry term primarily used to denote a "real PSTN phone number" associated with your VoIP account.

I have DID phone numbers associated to my VoIP account so that people on landlines and cells can call my VoIP phone. Without the DID, they can't reach me on my VoIP account. But, In-Network clients of a VoIP provider can all call each other by their Virtual numbers, or URI's for free. In-Network calling is almost universally free. And if someone on another VoIP network that has peering agreement with my VoIP network, they can dial my SIP URI can call me for free too (because it's all over Internet, just like email).

If you are using Free Call, then Free Call does not sell DID phone numbers for people to call you from landlines.