If this was a consumer transaction on a consumer card only a written dispute triggers the legal protections. See here:

http://ftc.gov/bcp/edu/pubs/consumer/credit/cre16.shtm

As a practical matter major issuers will extend the same protections if you dispute by telephone or web, but in a tough case you should protect yourself by following the rules precisely.

If you used a business card, or the purchase was for business purposes, the law does not apply. Again, most issuers extend the same protections as a matter of policy, but they have no obligation to do so.