what is the service call that lets me know who has tried to ring while i am on the phone

[Jupiter]

That would depend on how you have your call forwarding set up. If you have call waiting set up to give you notification that someone is trying to call you before it is forwarded to voicemail, then the number will be in your call log. Otherwise, it will go through to voicemail, where you should normally get an SMS notifiying you that a call went through to voicemail, along with the number (if they are not set to "private").