TL;DR : You don't tell them, you can't tell them, you need to carefully work around...
Whenever someone is coming to office / calling / e-mailing, (especially if arguing), I keep myself calm, stop talking and I start a very active listening. When the flow stops (and it will, at some point...), I use my self-called 'DIWA-RESC' method. Let's assume this is a call from a customer, asking why you took that much time to do the job, or why it's not done yet...
First of all, carefully listening to what she/he says, you need to determine if the person is:
- Disappointed / Interrogative / Worried / Angry
Then, we you know what your contradictor bears in her/his mind and what bothers her/him :
- Re-phrase / Explain / Solution / Conclusion
Customer : I don't know why it takes you that much time to have [ X ] done. We agreed on [ Y ], I should have gain [ whatever ] and now, I see that [ whatever ] ?! It's not fair, because [ whatever ].
Note that tone (and body language, if you face the customer) are very important tells, almost as much as words.
Now, you need to determine if she/he is more angry or disappointed or worried or interrogative.
Use only one feeling if you can.
The goal is: get a 'YES' after STEP #1. Not more, not less.
1 - Re-phrase : If I properly understood, you are [ fill the blank with the emotion ] because we had to [ X ], and that meant [ this ] to you ?
2 - Explain : (at this point, use impersonal if negative, personal if positive) We needed to do [ ABC ] in order to be on time. Unfortunately, we had to do [ D ] instead of [ E ]. That's a matter of [ security / quality / whatever ] -> Adapt to your needs / constraint.
Note: Your customer must feel like he's the most important for your business anyway AND Don't blame anyone either side AND Don't tell them what they're not supposed to know.
3 - Solution :
- Impersonal : according to [ law 123-1 / rule XYZ / policy A1B ], it is not possible to [ whatever ]. Therefore, it can only be [ whatever ]. That's why it had to be [ done that way / modified ]. I'm sure you understand there's no other way to do it.
- Personal : I can do [ whatever you best move / offer can be ] in order to [ whatever ]. Would it be OK with you ?
3 - Conclusion : thank you for your patience and understanding. Do you have another question ? Is there anything else I can do for you ?
It's important that your customer feels that you do your best, even if in your POV, he's not your #1 priority. It's a workaround, worth trying, and did great for me after practicing.
It's long, but I tried and keep it as small as possible, and hope I make myself clear here, if not, let me know leaving a comment, and I'll edit. ).