HMRC have issued confirmation that automated voicemail and SMS text messages received from HMRC are genuine.
The voicemails and SMS text messages are issued as trials on new ways to contact customers. As part of these trials you may be contacted by SMS text message. You will be asked to ring HMRC on telephone number 0845 300 3900. If you receive an SMS text message claiming to be from HMRC asking you to contact any number other than 0845 300 3900, you should not respond to the number but instead report the matter to your HMRC Contact Centre. This warning only applies to any SMS text messages from HMRC. It does not apply to any messages left in person by HMRC officers asking you to ring them back at your local office.
Further details are available here