The telephone symbolizes a line of communication and a catalyst for human companionship. The phone also symbolizes freedom, because it would allow Mrs. Wright to escape her home, if only through conversation. With no phone, she has no connection to friends or the community. The scenes when the phone is discussed are telling.

As Hale is describing how he found the crime scene, he says that he and Harry were on their way into town to sell potatoes. Hale adds that he had been considering a party telephone (sharing a phone line) with John Wright. Hale says that he'd suggested this to John Wright before but that John didn't want to participate because, as Hale quotes him, "folks talked too much anyway." This little scene is significant for a number of reasons. John's refusal shows how he wanted to keep himself and his wife isolated from other people. Hale adds that if he could address the party telephone idea to John and his wife, then that might sway the decision to accept the party line. Hale assumes (and it's safe to assume that Mrs. Peters and Mrs. Hale would agree) that Mrs. Wright would certainly want a telephone because she must be lonely in that isolated house. A phone line would give her some way of at least talking to friends.

Even if Mrs. Wright would have wanted the phone line, Hale assumes that Mr. Wright (ironically named) would not care what she wants. Hale says, "though I said to Harry that I didn't know as what his wife wanted mattered much to John . . ." This says it all. Even Hale knows how little John cares for his wife. The County Attorney dismisses this, promising to come back to it—as he does with other so-called "trifles." And this is another example of how the Sheriff and the County Attorney ignorantly dismiss all things related to the Wrights' relationship. Mrs. Hale and Mrs. Peters inevitably discover all the meaningful clues. This is one instance where a man (Hale) almost comes to discover something useful.

A stunned Mrs. Wright manages a laugh when Hale tells her a phone line was his reason for stopping by. Even in this mental state, her initial reaction is to laugh at such a thing. This is an indication that she knew John would never let her have the freedom of a phone. Mrs. Hale later repeats that she wished she'd come to visit Mrs. Wright. "Oh I wish I'd come over here once in a while. That was a crime!" With a telephone, Mrs. Hale could have called Mrs. Wright and vice versa. John's refusal to get a phone is just one more example of how he kept his wife prisoner and isolated from any other friendships or signs of affection from others.

In addition to the previous response we must also consider how the main character is literally inside and outside, isolated from society itself. The way she is treated as a woman in society and by her husband in private is a total disconnect from what life really should be.

The lack of the telephone also represents her inability to get help, or to be supported by the outside world. The only mercy she got was from the only other female character in the play, who probably lived her life the same way : Without support, and branded as a second class citizen. Since is a phone is an instrument of communication, the lack thereof makes the main character (like the previous poster accurately stated) isolated from humanity in a way.

In the play, "Trifles," a telephone, or the lack of one, is just one more element of Minnie Wright's environment that isolates her. 

She is a woman in a man's world, and is isolated from her society.  Her personality changes after she's married, and she does not lead a fulfilling life.  She is limited to her domestic duties and mistreated by her husband.  And she doesn't even have a phone to keep in touch with the outside world. 

Apparently, the only joy she finds is in a little canary, and when her husband strangles the bird, she strangles him.