The mobile phone giant is sending direct mail packs to 1.5m customers, designed as a puzzle book and containing information on how to use their phone aboard and details of its Relax Holiday cheaper minutes offer.
The campaign aims to show customers benefits of using phones abroad, such as keeping in touch with family and booking restaurants.
SMS messages will also be sent to pay-as-you-go customers with reminders of the offer.
The direct marketing campaign has been developed by Tullo Marshall Warren, and coincides with a TV, outdoor, press and online push to promote Relax Holiday.
Neil Minty, head of direct marketing at T-Mobile, said: "We believe the pack will help to promote our new Relax Holiday minutes as well as highlight other ways in which a mobile an be used on holiday."
Tim Leighton, TMW account manager, added: "Ultimately, we hope the campaign will encourage customers to consider using their mobile phone from Europe well in advance of them going."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .