The ratings of three Diversified Telecommunication Services stocks are down this week, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Premiere Global Services (NYSE:PGI) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. Premiere Global Services is a global provider of conferencing and collaboration services. The stock price has fallen 9.3% over the past month, worse than the 3.8% decrease the S&P 500 has seen over the same period of time. To get an in-depth look at PGI, get Portfolio Grader’s complete analysis of PGI stock.

The rating of IDT Corp. (NYSE:IDT) slips from a C to a D. IDT provides retail and wholesale telecommunications services and products, including pre-paid and rechargeable calling cards and wholesale carrier services. Share prices fell 13.6% over the past month.

For more information, get Portfolio Grader’s complete analysis of IDT stock.

Elephant Talk Communications Corp. (AMEX:ETAK) ratings are on the decline this week as the company earns a F (“strong sell”). Last week, it received a D (“sell”). Elephant Talk Communications provides voice, data transmission, pre-paid calling cards, and Internet service provider services. The stock receives F’s in Earnings Momentum, Equity, Cash Flow, and Sales Growth. For a full analysis of ETAK stock, visit Portfolio Grader.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.