The cheapest solution as earlier mentioned is use a wireless landline. Basta may 2G signal ang Smart sa area, mayruong PLDT wireless landline coverage. The drawback would be the area code kung walang specific area code provision ang PLDT wireless landline sa area ninyo kasi you would only be able to call toll free to any other wireless landline of the same area code.
Another costly method is via wifi and the use of SIP ata telephone adapters with fxo/fxs output/input.