The most recent credit if known. There is no credit on contract SIMs so it's not included.

In the UK, the PAYG credit can be included if O2 and Vodafone networks. That includes the virtual networks that use O2 or Vodafone:-

Vectone Mobile is a virtual network on EE, and the credit can be shown for mTrak units using this network.

The credit is checked by the mTrak after a text is sent, so if you have only just added credit, the new credit won't be shown.

O2 seem to show the credit about £1 low when GPRS is active.