You'd think that being nice would be a common human condition, especially in customer service, but the fact is that "nice" is hard to find these days and it's a shame, because I firmly believe that a "little sugar goes a long way" when it comes to delivering customer service.
For example, the other day I was in a chain hair salon waiting my turn for a hair cut. It frustrates me that it seems like the stylists always go on break right before my turn (and I believe I'm a decent tipper and a quick cut so I don't think they're trying to avoid me) and this last time was no exception. Just as it was my turn, she grabbed her cell and headed to the back room. I waited...and waited...and waited. By the time she returned I was frustrated, had almost walked out, and was determined to leave a poor tip. But you know what? She came back, apologized for the delay, and explained the nature of her phone call -- her younger brother, who was supposed to pick up her daughter from day care, was stuck in traffic and may be late picking her up. I could relate to that. I've had children in day care before and if you don't pick them up before closing time, the day care charges you extra for the delay and if they don't hear from the parent after a short amount of time, they call the authorities. It's not pretty and I completely understood. Total redemption.
Contrast that with a hair stylist who would have come back and, without a word, started to cut my hair. Big difference isn't it? And what did the first hair stylist gain by cutting my hair? A pleasant experience (came to find out her younger brother actually went to school with my daughter and I was facebook friends with her parents!) and a bigger tip. Good customer service pays.
So in a nutshell, try to take care of your customer right away, but if you can't:
1) Apologize for the delay.
2) Explain, if appropriate, the delay.
Some customer service professionals might scoff at the idea of sharing personal information with customers. You don't have to go into detail, a simple "Sorry, family emergency" or "Sorry, my boss wanted to speak with me" will suffice and will certainly be better than ignoring the fact you left your customer hanging. We are all human and we understand. No, not everyone will. but that's on them, not you.