PCMCIA modem devices are recognized as ordinary com devices and should be treated like a serial line with an external modem attached. To test this, some terminal program like kermit may be useful. Note that you may need to disable the internal COM port to get some PCMCIA modem cards to work.
Please see the download page for some optimized kernels & kernel config files.
For more information, please see the NetBSD Kernel FAQ and introduction to Basic NetBSD Networking.