When used as an adjective, hyphenated; when used as a verb, unhyphenated (dial up). Use as a synonym for dial-up is proscribed, as dial-up consists of outgoing calls (a network client), not incoming calls (a network server).
dial + in
(third-person singular simple present dials in, present participle dialing in, simple past and past participle dialed in)
When used as a verb, unhyphenated; when used as an adjective in networking or motorsports, hyphenated (dial-in).
dial + in, from tuning in to a radio frequency by changing the dial.