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Verify the connection
For phone line connected systems, check if you have a dial tone on your other phones or fax machine. If the other phone lines work, make sure nothing is unplugged on the payment terminal or at the wall. If the phone line for your payment system is shared with something else, check to see if there’s a dial tone on the other equipment which shares the line.
If your terminal connects via internet, see Why doesn't my POS work with the Internet in the Internet support section.