I just bought a Hyundai and was told by the dealer to expect a call about th service I received. Probably the best way to handle it would be just to answer once and ask that they don't call back. Most reputable companies will accept that.
I use Hyundai as well and gave the number when doing car service. All Hyundai users please threaten every customer service agent whenever they ask for our numbers and when we schedule service appointments in future. Tell them how irresponsibly they are using the information. Spreading a general threat and scare with all Hyundai service idiots may cause them to be careful in future.