I think I like that idea, but if I was going to enter just the start and stop times of calls
This is the problem with my model becuse I'm quite certain a number of people will forget to click the appropriate buttons so the quality of the collected data will probably be poor.

You are absolutely sure the phone company can't help you out? The certaily keeps loggs of all calls and the time they lasts how else are they going to bill you?

Well if you wish to have a go at it I would sugest you change the name of the macro buttons to something like "Call starts" and "Call stopped"

Then you get start time in column C as before (sheet and then you set a time heading in cell J5 and add a "Stop time" macro something like this perhaps?

Sub Call_Stopped()

Dim i As Integer

i = Range("Raw_data!J" & Rows.Count).End(xlUp).Offset(1, 0).Row

 Range("Raw_data!J" & i).Value = Time

End Sub
So you got the start time in column C and stop time in column J so by subtracting you get the time the call lasted and one probably need to add some error handling as well. Think about it and if you really wish to have a go at it I'll add what help I can.

Alf