"Phoning it in" is an expression for a thing which is generally done half assed.
The implication being that an individual cannot be damn well bothered to make a personal appearance at a given function when a show of support (whether actual or merely representational) is requested or required. Instead of appearing in the flesh, one instead sends one's regrets by telephone as a sort of token gesture.
While there are a few instances where phoning it in may be a legitimate or understandable course of action, the expression is generally considered to be pejorative.
In the age of conference calls and call centers, phoning it in may be used to simply refer to a person on the phone whose attention is somehow divided or compromised.
The exact origins of this expression are unknown, but it is likely that it was coined by someone involved in the theater community. After the telephone became ubiquitous, the joke amongst stage actors was that an actor would be given a role so minuscule that one could simply stay home and phone it in.
The first recorded usage of this was in February 1938 edition of the Detroit News in which Thornton Wilder's Our Town, a new play at the time, was criticized. In addition to its content, Wilder's play aroused controversy for the emphatically bare stage instructions of "No curtain. No scenery." In a column by one Senator Soaper (being the pseudonym for Harry V. Wade) the comment was made that since "a Broadway drama has attained hit proportions with no scenery, the next step is to have the actors phone it in."
And thus a meme was born.
Senator Soaper would later repeat the quip in reference to FDR by writing: "As far as we know, there is nothing in any rumor that in case of a fifth inauguration, F.D.R. will phone it in."
By the mid-1950s, the meme became fully enmeshed in the cultural consciousness.
The concept quickly crossed over to the world of radio, where important, overly busy folks who could not make it to Radio City Music Hall to weigh in would do so over the phone. In the unlikely event that the original expression were used in such a case it is likely that it was intended in a non-disparaging manner. More likely the "it" would be dropped entirely: Ike phoned in.
At some point the meme mutated to "mail it in" upon crossing over into the world of spectator sports. The exact rationale for this are unknown. It may simply be due to the fact that having an full exchange of ideas by parcel post can be a process that takes days.
Fast forward to 2005 when nearly every individual has a cell phone and only corporations and old people have landlines. Enter Ned Ludd, writer and enterprising college radio DJ with a keen sense of irony and a shortage of studio space. Ludd has the bright idea to have nationally touring musical guests appear on his radio show via telephone. Each episode of his program consists of entire performances and brief interviews conducted via a regular landline telephone (landline have better sound quality than cell phones). Like Dial-A-Song, except on the radio. The show was frequently qualified by the assertion that Lo-fi is the right fi. Fittingly, Judd's first guest was The Mountain Goats.
Jude was a DJ with KDVS, based out of Davis, CA but the program was such a hit that many other college stations ran it in syndication. Sadly, that program seems to no longer be active as Jed has admitted defeat to the slow eradication of landlines and since gone on to greener pastures. Thankfully, all 557 shows in the archive remain online (alternately, hear here) and he does maintain a FaceBook group in which he charts the various musical projects which once appeared on his program, as well as recent material that addresses similar themes.
More recently the term "phoning it in" has been applied to musicians who record an entire album using their smartphone. One such effort was done in 2011 by the electronic muscians Nuclear O'Reilly. They claim to be the first to use an iPhone to create and record original beats and make music. You can hear their result here.