Assume you are talking about a 7937. Max calls in 7.xx is 6. If you look inthe programming of the extension, go almost all the way to the bottom of the page, you will see:
Multiple Call/Call Waiting Settings on Device xxxxxxxxxxx Note:The range to select the Max Number of calls is: 1-6
Maximum Number of Calls
Busy Trigger (Less than or equal to Max. Calls)