Fongo powers free calling solutions for a number of brands. This forum includes information and tips on all of those offerings. Note that your specific services may vary based on the specific Fongo product that you're using.
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Most of the standard ones should work ( not from your rotary phone though ) but it will be dependent on your ATA. Have a dig through the manual and that should tell you. You can usually customise them to suit from within the ATA.
I know some definitely work because a common question to answer here is when someone turns DND on by accident and no longer receives calls.