I got flamed by the sysadmin of a site. Calling an ISP voice is one of the best ways for dealing with spamming problems involving their site. For one thing, a phone call will get an ISP's attention much more than an e-mail will as phone calls generally take more time. Any legit business will always take the time to listen to complaints against them. When calling an ISP voice, you should be sitting in front of your system with all the necessary information handy before you place the phone call.
Nothing frustrates an anti-spam ISP more than a clueless complaint.
Keeping calm and collected also cannot be emphasized enough. If you call up an ISP and start yelling at them, they will brush you off as some sort of lunatic instead of taking your complaint seriously. It can also have the negative side effect of having an ISP start to ignore complaints, which is a bad thing. This is so that the telephone companies will know where the 800 number calls are coming from so they can bill the owner for the correct amount. This is different from Caller ID in that you can not block your own number from being sent.