If you were travelling on the eastern suburbs line in Sydney, that would be why. It was the last line to join the underground transport network's mobile coverage upgrade that begun last year.

Commuters can now make calls, send texts and browse the web while hurtling through tunnels after Optus completed the last phase of a multimillion-dollar project to provide coverage in one of Sydney's most annoying blackspots.

The last phase of the upgrade involved the rollout of 18 kilometres of "leaky" cabling.

Commonly used to establish communications systems in mines, a leaky feeder is a length of cable that has had gaps or slots cut into it to allow signals to "leak" out, providing better mobile coverage. Similar systems are used in London's Underground and Hong Kong's Mass Transit Railway.

"Last year coverage went live in the City Circle and tunnels north to Chatswood, and I am pleased we have now put an end to mobile phone dropouts for customers travelling south to Erskineville and east to Bondi Junction," NSW Minister for Transport Gladys Berejiklian said on Thursday.

She blamed the previous government for the delay in getting coverage into tunnels.

"It is unbelievable it has taken this long. The limited coverage through tunnels has been a frustration to our customers for many years," Ms Berejiklian said.

"Optus first approached the then State Rail Authority in 1996 to install infrastructure to provide mobile phone coverage in the Sydney CBD underground rail network. But Labor failed to do anything about it."

The project also involved Telstra, Vodafone and RailCorp.  

"It's been a complex project, ensuring continuous coverage along Sydney's train lines and tunnels, but we know how important it is for our customers to be connected anywhere, on any device, and in particular get the most out of their time spent commuting,"  Vic McClelland, managing director of Optus Networks, said.

smh.com.au, with Matthew Jones