Vast distances, loneliness, wind and tide, are no obstacles to technicians answering the call, so that others can, well, answer their calls. China's largest cellular network provider is boosting and ensuring 4G signal access in the South China Sea.

Their work has connected many isolated remote islands to the outside world. Even Vietnamese fisher-men, who illegally fish near, have bought the ChinaMobile SIM cards for signals, according to local islanders.

But the scale of the operation, including shipping, building, maintaining and upgrading transceivers and cellular radio towers, is huge, especially as it takes 10 to 15 hours to cover more than 300 km from Hainan Island to Yongxing Island, part of the Xisha Islands.

"Sometimes we have to relay a signal if we are to reach a more remote island, transferring from a bigger ship to a smaller one, and so on'', said Su Xun, a veteran technician with China Mobile's Hainan branch. "Sometimes we rent a boat from local fisherman for shipping."

With the distances involved, time is of the essence. Su noted that the government ships often stay at a destination for just a few hours before lifting anchor and sailing again. This means that all the work, be it tower installations, off-loading equipment, must be done as efficiently and quickly as possible. Any delay can mean weeks of waiting.

"Sometimes it is impossible to finish fixing or updating the equipment within three hours. We have no choice but to stay for more than a month to wait for the next ship to dock," Su said.

Deng Ruming, a colleague of Su, works on optimizing the networks and he noted one frustrating incident to highlight the point.

"The laptop is a must for tuning and upgrading transceivers. Once a colleague's laptop went overboard during a storm,'' Deng said adding that the trip had to be made again. Thankfully, these types of incidents are rare.

Other challenges facing Su and his team include high levels of humidity and corrosive seawater, both of which can play havoc with the sensitive equipment.

Those on board "Sansha 1" - the largest and most advanced vessel commuting between Hainan Island and Yongxing – have reason to appreciate the technical expertise that allows them to utilize 4G signals at any time throughout their trip and make a call at their convenience.