Do data-only SIM cards have a phone number assigned to them? Some providers offer a "data only" sim card. Do these types of sim cards have a phone number assigned to them? If yes, what happens if a dial out is attempted of if there's an incoming call?
A number has to be assigned so that the (rather old) system can recognise the SIM.

Just because it has a number though doesn't mean that any calls can be made or received and will already have been disabled by the network.

The provider can add services as wanted/needed though: SMS can be useful for managing routers.