TET is responsible for the free internet service provider's IT infrastructure and phone lines in its head office in Hanwell and internet café in Twickenham, which is currently offline due to unpaid bills.
"We notified MyISP that its licences had run out, but having received no payment, we had to suspend our services and issue court proceedings," said a spokesperson for TET.
Adnan Awan, business development manager at MyISP, blamed lack of investor commitment as the reason for the company's troubles.
"Our investors have been abroad and none of the staff here are authorised to write cheques," he said. "They are back now and I believe some of the money owed has been paid, although the internet café is still offline."
Awan added that some of the £700,000 it is claimed to owe TET is debt belonging to its investors' other business interests. He said that MyISP is due to meet with investors this week to discuss the appointment of a new managing director and iron out the problems.
MyISP has been beset by problems, which led to the loss of key staff earlier this year. It has now also lost acting operations manager James Young.
Young stepped in following the departure of co-founder and chief operations manager Simeon Quarrie in April, amid disagreements over the company's direction.
Internet development manager Daniel Rueppel walked out in July, blaming lack of management control.