The first fifteen seconds are the part almost everybody rehearses and dreads. Once you are through them the conversation generally carries itself, because the other person starts asking questions and you only have to answer.
So the useful preparation is not a full script. It is a first sentence, and knowing what you want before you dial.
Openings that work
Every one of these opens with the problem rather than an apology, and ends with a question, which hands them the next move.
Pick the one you are facing
“Hello, I have been charged for something and the amount is not what I was expecting. Can you look into it for me.”
“Hello, I am following up on something I was told would be resolved by now. Can you tell me where it has got to.”
“Hello, I would like to cancel my account. Can you tell me what you need from me to do that.”
“Hello, something has gone wrong and I would like to explain what happened. Can I go through it with you.”
“Hello, I am trying to sort something out and I am not sure I am doing it right. Can you point me in the right direction.”
Say what happened, once, in order
Whoever answers can only help with the actual sequence of events. Give it once, cleanly, then say what you need. Leading with the ask before the facts almost always makes the call longer.
Two sentences is usually enough. What happened, and what you would like to happen now.
Four things before you hang up
This is the part that saves the second call.
4 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
You do not have to be firm
A great deal of advice about difficult calls is really advice about being assertive, which assumes the problem is that you are too soft. Usually the problem is capacity, not confidence.
Being polite and specific works with almost every call centre, because the person answering has a fixed set of options and is deciding which to offer. Clarity about what you want moves that further than firmness does.
If the call does not resolve it
That is a normal outcome. Plenty of calls end with somebody else needing to look into it, and the reference number is what makes the next one continue rather than restart.
If the barrier is getting to the call at all rather than the call itself, that is a different problem, worked through in making a phone call you have been avoiding.