If you have good internet you can use an inexpensive device called an Obihai and move your number over to be conducted through Google Voice. I've been using this for years and it works flawlessly.
You won't be able to port your landline number over directly however because Obihai only transfers cell numbers. You can get around this easilyhowever by buying a throwaway cell, transferring your existing landline number to the throwaway, and then from that cell to Obihai.
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