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.