Hola Rodrigo,
i can't really vouch for the v.18 reliability, but as it comes to FS in general, this is the most advanced multi-purpose PBX on the market(even though it's not for sale:)). Comes with amazing customization capabilities and a superb support channel. There's a learning curve which is not nearly as steep as alternative solutions. I can see Steve Underwood as the author of v.18's mod_spandsp extension, so i think he can drop a line or two. FS is know for being super-strict when it comes to standarts implementation.
Cheers