Yes. You need to allow remote forward for your extension through system programming from x10/x11 with the code #322. Feature 00, Left Intercom Button twice, #322.
Then, you need to save your cell phone number in speed dial 80 of your extension. Go to your extension, press feature 00, feature 80, then enter your phone number. You can test this by hanging up and pressing feature 80 and make sure it calls your cell.
Finally, to forward it, you will press Feature 11, then enter your extension number followed by 80. This is forwarding your extension to speed dial 80. To cancel the forward, press Feature 11, then your extension twice. Essentially forwarding your phone to itself.
Technically, off-site forwarding is not supported with POTS lines as there can be voice degregation on the lines when you connect them together, but I have many clients successfully using this feature.
Also, only calls specifically transfered to your extension will forward off-site.