calling someone after their scheduled time should be a common sense no-no. You're giving the user X minutes to do something. You need to be available during those X minutes.

Without knowing when the previous call was scheduled it's hard to know whether it was a case of scheduling two appointments too close or if there was enough time but the previous person got chatty. Either way though if it looked like the original rep wasn't going to be able to make the next scheduled call someone else has to be available to fill in... or the rep has to get the previous person off the phone by letting him/her know he has another person to assist and the process is time sensitive. The latter is tricky because then you can end up with the previous person feeling slighted/rushed.