Funny, two telco complaint threads in one day... I'll make sure I don't paste any "naughty" links on this one.
First, glad you're in Australia with me where things make sense.
Step 1: Collect your information. I'm talking dates, times, names of people you spoke to and so forth.
Step 2: Call them and talk to them. Try to keep things calm and emotionless. 9/10 operators will transfer you away if you're "mid way going off at them". Remember that often they'll try to help straight away, but if you give them any problems they'll probably just cut you off. Explain the situation rationally and see if it can be fixed.
Step 3: If you get no luck from the operator you speak to, ask to speak to their supervisor. Remember to keep calm and civil. Try not to get frustrated. It's red tape that they hope you can't be bothered dealing with.
Step 4: If the supervisor cannot assist, then ask to make a formal complaint. Most companies have internal dispute resolution services. They may request the case be put in writing. Ensure that everything received from them, and everything they receive from you, is documented somewhere. Also, many companies keep a file of what discussions they have with customers. Privacy laws in Australia allow you access to this information. Ask for a copy of your initial contract, and any notes regarding your past dealings with the company. Most of these complaints departments are flexible, because they do not want you going to step 5
Step 5: If the company cannot resolve your dispute internally, then you have recourse with the telecommunications ombudsman. In Australia all matters referred to them incur financial penalties from your telco provider. Often they will do what they can to make the matter go away rather than have you go to the ombudsman. If you need to take the matter further, go here:
Telecommunications Industry Ombudsman
I'm not sure which provider you're with. All the majors in Australia are in the TIO member list; Vodafone, Telstra, 3, Optus etc.
There are plently of templates for complaint letters on the web.
The main thing is to distance yourself from the emotion and frustration, as that makes resolving your complaint more difficult. You telco not only needs to fix the issue, but also calm you down. Act as if you're doing this for someone else. Be courteous, polite, understanding of the limitations placed by the company on the person you are speaking to, and don't be aggressive. In return, be assertive, honest, and clear about your issue. The people you speak to will appreciate that more than anything, and that in turn will probably make them more likely to go the extra mile to help you rather than transferring you into limbo like they did the crazy guy who called in earlier and wouldn't stop yelling at them.
Hope this helps.