I am writing a formal business email to a client, and at the end I would like to ask him when would be his most convenient time to have a phone call. The sentence goes like this:
请您让我知道哪个时候您最方便接电话。
I am not sure if it would be better to use 什么时候 instead.
If someone could please advice me on what subtle differences there are in the meanings of these two phrases, and suggest which to use, that would be very much appreciated!