Mobile operator EE has been landed a £2.7m fine from telecoms regulator Ofcom for a series of billing errors that led to tens of thousands of its customers being overcharged. The operator incorrectly charged subscribers for calling its customer services number while roaming in the EU.

A series of mistakes meant they were charged £1.20 per minute for calling its 150 customer support number, instead of the standard 19p rate. EE also continued to bill customers for calling the number after making it a free-to-contact service in November 2015.

As a result, EE overcharged approximately 40,000 customers amounting to a total of £250,000. The operator has blamed the excess charge on a "technical billing issue" which it claims affected less than 0.1% of its subscribers.

How to check if you have been overcharged by EE & what to do

EE said it had contacted "the majority" of customers who had been affected to apologise and provide a refund. Previous EE customers who were with the company between 2014 and 2015 may also be eligible for compensation, although they will have to act fast as the operator is donating unclaimed excess fess to charity.

Particularly if you are a previous EE customer who has left the network since the issues occurred, you should check your old bills — provided you kept paper or email copies — to see if your calls to 150 from abroad were charged at £1.20 per minute instead of 19p. You are also due a refund if you were billed for any calls made to 150 after 18 November 2015.

EE has also created a freephone number for any customers who are no longer on an EE or T-Mobile price plan and need to speak to the company. If you call 0800 079 0216, customer service advisors will be able to verify if you were impacted by the error and whether credits have been applied Failing this, you can call EE's customer services team on 0800 956 6000.

An EE spokesperson said: "We accept these findings and apologise unreservedly to those customers affected by these technical billing issues between 2014 and 2015. We have put measures in place to prevent this from happening again, and have contacted the majority of customers to apologise and provide a full refund.

"Following Ofcom's findings, we have made a number of additional improvements to our systems and policies to allow us to better support our customers in the rare occasion that billing issues do occur."