If it is a standard telephone type cable, then it should be ok. Some have 4 pins, others two. If your modem has 2 pins then either cable will be ok. If the modem has 4 pins then it will require a 4 pin modem.
Some PCMCIA have special connections for the cable - if that is the case with yours, then you will need to check with the manufacturer site.
As long as the cable fits, the worst that will happen is that it will not work; it's not likely to damage the modem.