Cross-world Tells already work. If it didn't then how would you message groups on Party Finder from other worlds. Tells don't work if someone is in an instance, so I assume the people you tested it on where in an instance or in a zone they couldn't be contacted from. Just gave it a go and was able to /tell someone on Phantom when I'm on Spriggan.

Cross-datacentre Tells would be nice however it's still new so that may be a feature in the future, along with making Party Cross-DC.