M1, StarHub and Singtel suspended all mobile services along the Circle Line for about two hours yesterday, as the authorities investigated delays on the MRT line.
At about 8.50pm yesterday, the three telcos said on Facebook that they were cutting Internet access and voice and text message services.
At about 10.20pm, SMRT tweeted: "Tests have been completed. Telco signals will progressively resume along Circle Line."
SMRT had alerted commuters that there may be delays on the line because of " intermittent signal interference". It also warned of the temporary suspension of mobile services.
On Wednesday, there were delays after the same "intermittent signal fault" on the Circle Line persisted for three hours.
M1, StarHub and Singtel had alerted their users that they may suspend services the next day, should the same fault occur.
"This is at the request of Land Transport Authority (LTA) and SMRT to facilitate investigations," StarHub said on Facebook.
The unknown interference resurfaced on Wednesday morning, after a two-month reprieve.
It first appeared in late August and crippled Circle Line services for about a week. It disappeared as suddenly and as mysteriously as it appeared, and engineers were not able to trace its source.
The stray signal interferes with the wireless signal communication between the trains and the tracks, causing the trains to activate their emergency braking systems.