I am glad that you understand how your response to cold calls can affect your company's brand. I would like to offer some tips.
First, understand that they are just doing their job. Just like us, they want to bring in the sales too so we need to empathise. There is no need to be rude to them. I always believe that we can say 'no' in a positive way.
Once you realise it is a cold call, ask them to state their intentions quickly. Don't let them go into their sales script.
Acknowledge the good in their product/service (even if you don't have a need for it). For example, "I am sure your product is good at what it does."
Then, tell them why you don't need their product/services. There is no need to have lengthy explanations. You are not obliged to anyway. You can say something like: "We don't have a need for it right now" or "It is not relevant to what we are doing"
Then wish them well with "Hope you find the customer that is good. All the best" and end the call.
If they are pushy, just interrupt them and repeat politely. For example, "I have to interrupt. I don't have a need for your product. So I have to end this call. Wish you all the best."
They usually will say, "This will only take a minute." You can reply with "It is not relevant to me, so I can't offer you the time. Wish you all the best." and put the phone down.
Always end on a good note by wishing them well and keep your tone friendly. All this can take less than a minute and you only need to say less than 10 sentences.
Many people will pretend that they are in a meeting. I find this pointless because they will only ask you if they can call you back at a later time.