Some provider woes: FAX over VOIP is a PITA. I've not yet seen an ATA or softswitch that handled it reliably.
E911 for mobile devices sucks. Regulations, and the E911 system, do not seem to have the flexibility for handling this in a seamless way. Call routing (on a more global scale) sucks. Keeping calls pure IP is sexy, but the routing protocol for it is nonexistent (and please don't say ENUM).
