Commuters who took the Circle Line yesterday found themselves unable to call, text or access the Internet inside trains for almost the entire day.
That was because all three telcos - Singtel, StarHub and M1 - shut down access to their mobile services while tests were carried out to isolate the cause of the signalling faults that have affected the line since Wednesday morning.
The networks were first suspended for about two hours on Thursday evening to allow investigations into the faults.
But at about 7am yesterday, rail operator SMRT, in a tweet, warned commuters using the Circle Line to expect "temporary suspension of telco signals".
Initially, mobile phone services were supposed to resume along the line at 9.30am, but the Land Transport Authority (LTA) extended the suspension to continue the tests.
On its Facebook page, LTA apologised for the inconvenience but explained that the suspension was required to "fully investigate all possible causes" and "the information gathered will be studied".
It added that the mobile network will be progressively restored from 11am today and is expected to be fully up by noon.
Some Circle Line commuters experienced brief delays yesterday.
Sales executive Aaron Chong, 30, was travelling to Bishan from Tai Seng at about 10.45am when the train stopped at Bartley station for about six minutes.
Mr Chong said he was unaware of the mobile service suspension as he rarely takes the Circle Line.
"It's inconvenient if I have to get in touch with clients or if I'm expecting an urgent call," he added.
At Buona Vista station, a commuter who wanted to be known only as Rae G. shrugged off the mobile service suspension.
"It's not like there is no service islandwide. It's a minor inconvenience," said the 17-year-old Anglo- Chinese Junior College student.
The mysterious signal interference first disrupted services on the Circle Line for a week in late August, resulting in slower train speeds and reduced service frequency.
It resurfaced on Wednesday after a reprieve of two months.
In a joint statement in September, LTA and SMRT said they were installing "spectrum analysers" to detect the presence of interfering signals.
They also said they were considering various other measures to prevent the problem from resurfacing, including fitting trains with electromagnetic shields and changing the signal frequency of the network.