There are innumerable instances where it would be a waste of time and money to consult a $50 telephone attorney as you suggested (or any attorney, for that matter). Making a decision on whether or not to sell usage rights to a “branding icon” being one of them. If I contacted an attorney each time a decision of this sort needed to be made, half my income tax deductions would be for covering needless legal fees.
This isn’t a legal question unless one has made the decision to sell only usage rights and needs advice on doing so or a contract written up.
In this case, though, it would be far better to just ask the clients if they wanted to buy the illustration outright or just buy limited usage rights, then figure out the fees involved.
If it were me, I would, of course, advise the clients to buy the icon if it’s intended to become part of their brand so they did not risk me reselling part of their brand equity to others or raising the licensing fees down the road.
Of course, if it were me, the dilemma would never come up since I would just sell it to them. Whether one terms it a logo or a “branding icon” makes little difference. According to the original poster, it’s a line illustration of the principles in the company to be used as part of their brand.
If upon further client questioning, it turned out they just wanted the icon for, say, a small limited run advertisement, that might or might not run again next year, fine. Simple usage rights would be a possibility, I suppose (although I, personally, still wouldn’t go there). But this is still not something to ask an attorney about; it’s something to talk over with the clients to see what their preferences would be.