Do they require a Deskphone or happy with a softphone?
I have also configured a SIP softphone (many free ones out there) on a PC that has a VPN to the office network. That will get past the firewall issue but it does affect the audio if the bandwidth is not good.
This is more a networking question than a phone one to be honest.
generally speaking, that's a decent solution. And you are correct, VPN's aren't great for voice traffic due to their underlying technology but it can work fine, just need to try it out and see. Nothing to lose. There are several free softphone's that I've used against mivb.
