Unfortunately, that's not possible with IPMA at this time. You do have a concept of a primary assistant who handles their calls whenever they're available, but when the primary assistant isn't available, the selection of a backup assistant is more or less random. When we first started deploying IPMA, we and our customer filed enhancement request CSCeb20786 on this topic (among others). I have no indication that a fix is roadmapped or intended to be addressed soon.