My answer would depend a bit on what your role and the customer's role is, but I'd go with something like:
"Dear Customer,
Just confirming our meeting for (date) (time). In order to make most productive use of our time, would it be possible for you to ensure that there are no interruptions during the meeting?
Many thanks,
Me."
If this is the first time you're mentioning the issue to them, then I wouldn't take it any further than that. If there's a recurrence of the issue after you've raised the issue, that's the time to escalate the issue.