Hi
It doesn't need to be a PCI modem, but I would prefer one because everything can be nicely tucked away than and it's less wiring. But if only oldskool external serial modems do the job I'll use that then.
My preference is MultiTech. IIRC, many others on this list have also preferred them.
Multi-Tech also has a line of PCI modems that work quite nicely. I have on in a Dell Itanium server using qpage to act as our notification pager for Nagios using a TAP-to-SMS gateway.
Works like a champ. noe that this is PCI and not PCIexpress. A lot of new servers do not have a pure PCI slot in them anymore.