Adding emotions such as smiley faces and other icons to text messages can double the average bill of most mobile users new research has shown.

A major factor in the price hike is unknown to most phone users: the bullet-point mark cuts text message size-limits from 160 to 70 letters.

This means texters including the bullet point may actually be sending two messages rather than one. And small picture icons - known as emojis or emoticons - have the same impact, Fairfax reports.

Melbourne resident Toby Passauer uncovered the issue after including a combination of punctuation marks to end his messages with a big-nosed smiley face, like this: =●)

He ascertained the bullet point was splitting one long text message into two 70-character messages.

"When comparing the bill to my phone, I noticed that [Telstra] were double charging for single texts," he said.

"I went through the whole bill and found it over and over again. I started noticing a pattern on the messages that were double billed - I had used the bullet point."

Neither Telstra, nor the iPhone maker would take responsibility, Mr Passauer said.

Investigations by Fairfax have found that global standards on short-message-service (SMS) are responsible for this confusion.

A Telstra spokesman said SMS character limits varied depending on encoding.

But consumers get no warning that some symbols will cut their texts in half, unless they keep a close eye on the character count.