Since 2011 the Government department has spent more than £40,000 dialing the service, which was first introduced in Britain in 1936.
Last year it was reported that the MOD had brought in a ban on ringing 123 and staff had been informed of websites they could use to check the time instead.
But even with the ban, the final bill for 2012 was £18,804 and more than £6,000 has already been spent this year.
Added to the £15,162 the department spent in 2011, staff at the MOD are so keen to check the time that they have made more than 130,000 calls at 30p each during the last two-and-a-half years.
An MOD spokesman said: "A ban was introduced to our newest telephone network, but due to a technical error with some IT servers there has been some inadvertent spending on the speaking clock which has now been stopped.
"Overall spend on this has decreased from more than £15,000 in 2011 to just £6,000 this year since the ban was introduced."