Consumer smartphone plans are back under the spotlight after successive changes to the way they are billed.

Earlier this month, plan changes by value mobile virtual network operator (MVNO) Amaysim cast light on a recent move to make prepaid plans expire after 28 days instead of 30 days.

Amaysim is one of the last telcos and MVNOs to make the shift to a 28-day billing cycle, but their move was heavily publicised.

A 28-day billing cycle means telcos can effectively bill customers for a 13th month each year.

Now, a new report by two UTS IT engineers for the Australian Communications Consumer Action Network (ACCAN) has shed light on the impact of another billing practice known as “megabyte rounding”.

Though some telcos have moved away from megabyte rounding, ACCAN notes there are still plans in the market that use it, and there is likely to be a legacy user base stuck with the controversial contract term.

Megabyte rounding sees smartphone data sessions rounded up to the nearest megabyte, rather than charging the user only for the data they use.

ACCAN funded an investigation of the practice and found that the median loss of data through the “megabyte rounding rip off” was 23 percent.

“This means that if you are on a plan with 5GB of data per month, you might only get to use about 4GB before it runs out,” ACCAN said.

The situation is even worse for light internet users.

“Those who use less than 500MB per month were the most significantly affected by the use of megabyte session rounding and typically added around 80 percent of their actual used data to rounding ‘overhead’, the research noted.

“Megabyte rounding will have the biggest impact on consumers who are low data users,” said ACCAN CEO Teresa Corbin.

“This is data that consumers are paying for that they don’t get to use. Consumers shouldn’t sign up to these poor value megabyte rounding plans.”

The UTS research is based on 142 mobile users that contributed “useful itemised billing information – which included more than 20,000 data sessions”.

The researchers noted that session handling processes by the telco and the smartphone device could have a large impact on how data sessions started and stopped – and therefore how much data is lost to rounding.

They noted it could be valuable to better understand these processes in order to preserve quota that users had paid for.

“The session handling has a large impact on the session rounding overhead and may have a significant effect on the consumer's data bill,” they said.

“It is a complex function of handset hardware and software configuration, carrier policy, terrain and usage pattern which defines when sessions are terminated.

“For example, the one device may end a session more rapidly than another in an effort to reduce power consumption.

“This behaviour is not well understood and provides a significant research opportunity to minimise both consumer data costs and device energy use.”