Any "unlimited" service suitable for callback?

[RTuesday]

Is there any "unlimited" VOIP (or other) service that allows callback or perhaps conferencing to get a connection between two PSTN phones with no per-minute cost? For a monthly fee. Web or phone call setup.

The catch: one of the phones would be a Mexico landline. The other end would generally be US or UK. By "unlimited" I mean one or two calls a day typically to a few specified numbers, so a big bundle of minutes might do it also. Key thing is both ends are PSTN phones, not IP.

This isn't for me, but for some friends of mine in Mexico.

[prion]

Which city in Mexico?

[RTuesday]

Originally Posted by prion

Which city in Mexico?

Mexicali area code (+52 686). I know there are some tariffs which are limited to Mexico City, or perhaps the big three cities. But this would need to be a tariff that covers landlines in more/most of the country.

Looking around it seems like I can get the call cost below 4c/min (Mexico landline to US/UK) with callwithus.com callback, which is more than cheap enough for a low user like me. Would be nice to find something flat-rate for those who need it.