[Ben B]

What Stephen meant was not to actually change networks, but is your phone set to automatic in terms of network choice as Virgin Mobile uses the T-Mobile and Orange networks and will automatically switch to Orange signal when there is no T-Mobile (Virgin) signal. Also make sure your settings allow roaming as sometimes phones don't allow the switch to the Orange network as it thinks you will incur extra charges. Have you tried your SIM in a different phone such as your sisters to see if you get signal indoors on that? Try manually scanning for networks indoors too and see if the options for Virgin or Orange (sometimes virgin with small v) are available to select.