I'm not sure exactly what you mean by "tried it for Google Calendar",
but the key point here is that Google Calendar is an on-line storage
repository for events, not for phone numbers, so it does not sounds
like a good fit for your use case.
Have you taken a look at Google Base? It's intended more as
repository for structured or semi-structured data, potentially using
user-defined formats. It also defines a richer query language that
enables queries based upon the attributes of the data. Google data
API support for it has not yet been made available, but in general it
sounds like a possibly closer fit for what you are trying to do.
-- Kyle
Google Base now has GData API suppport too, just announced today here:
http://google-code-updates.blogspot.com/2006/08/new-gdata-api-google-base.html
Hope this helps!
-- Kyle