When phoning clients and your contact is busy, always ask the person who answered the 'phone if they can help. When calling clients, never assume that they'll have your job sitting on the desk in front of them. 99 times out of 100 they won't. If a client asks you to overtype a file, make sure you have compatible software. When invoicing your clients, please try to do so promptly, at least within a week of completing the job.
Invoices referring to jobs which took place several months ago cause accounting problems.
When sending e-mails to a client, it's always a good idea to confirm via telephone or fax that you've sent it. Sometimes files do go astray and the sooner we know there's a problem, the better. Unless you need to speak to someone urgently, try not to call the office first thing in the morning or last thing in the evening. The thing you will notice all the above have in common is that they end up saving time for the project managers involved. But a little consideration can go a long way.