I had previously worked with a Czech head chef at a well known Sydney restaurant and tthe Operations Manager invited me back to work with the sommelier who also had to answer the phone during busy periods ( as he was behind the bar a lot) and try and deal with the demanding VIP customers ringing with all sorts of requests.
It was noted one evening that he was slurring his words. (Later he confessed to me that he doesn't always spit so had had a few sips of wine!) The phone rang and the boss overheard him on the phone 'slurring' his words a little. They decided that he needed a little 'elocution' type work as they really appreciated his work and wanted to help him.
As you can imagine this is a very sensitive topic and difficult to approach the person who needs help. This case was no exception so we didn't get off to a good start but being mature adults we quickly ironed out the problems and had a very successful three by two hour session course
The initial brief was that he slurred his words and didn't speak clearly over the phone. I decided we needed to work on final consonant sounds and linking. Then on meeting my client included chunking, pausing and pacing.
By the end of three sessions and much practicing at home and a LOT of recordingand playing back ,y client ad developed a beutifully clear 'telephone voice'.
I must give credit where credit is due and acknowledge a very good site:
http://sbinfocanada.about.com/od/speakforsuccesscourse/a/speechlesson1.htm