Fix voice mail number to modify for some carrier card.

1.H3G Denmark SIM Card is set to "not available" in UST(Usim service Table) to
  EF_MBDN file of SIM CARD. so do not edit voicemail number.

2.AOSP default voicemail number is empty.
  so if inserted H3G Denmark SIM card,
  User can't edit voicemail number and do not use voicemail service.

Bug: 14093735
Change-Id: I2469ef8e8bda376adcdc9549bf4227bc4c0ddb2e