If you're talking about Sexually Transmitted Diseases, and you're in the US, here is a local STD hotline for all states...and it's toll-free, just call 1-877-889-5407.
If you're talking about Subscriber Trunk Dialing and asking "what is the telephone number of this phone?" then the only general advice I can give you is to first look to see if there is a label on or near the telephone with the number, or otherwise call someone who has Caller ID and ask them to read back to you the number you called from.
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