Any "unlimited" service suitable for callback?
 

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.

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.