Isles MSP Tavish Scott says major questions need to be answered following a phone coverage blackout which cut off the emergency services and closed Sumburgh Airport.
Yesterday the airport closed at lunchtime because of the phone line problem affecting its radio transmissions.
Eleven flights were cancelled and one diverted to Kirkwall.
Flights were back on schedule today and flights were put in place to clear the passenger backlog.
BT said blown rectifiers and fuses at Wideford Hill radio station in Orkney had been the cause of the problem, affecting both Shetland and Orkney.
HIAL said the issue was resolved at Sumburgh at 6.45pm last night.
As well as the airport, people across the isles lost mobile phone and landline signals for several hours, and the fault disrupted the emergency services, with Gilbert Bain and the coastguard both being cut off.
Mr Scott said it was “completely unacceptable” for Shetland to be without emergency communications.
“If the reason was a blown fuse at a BT Orkney transmitter then why was there no back up? The NHS advised people not to phone 999 but instead go to Lerwick’s Gilbert Bain Hospital.
“The coastguard had no radio or phone communications to the Scottish mainland and flights into Sumburgh were suspended.
“Shetland was in short cut off because of a fuse.
“This is the 21st century. There has to be a back up. Shetland will expect a full explanation from BT. We will want to know what the emergency services and the NHS are now doing to ensure they do not depend on a communications system that failed.
“There are major questions to answer. I will also be raising this with the Scottish Government given their responsibilities for emergency service cover.”
Yesterday BT apologised for the break in service and said it was under investigation.