48 is a virtual network operating on the 3 network.
Because of this some phones think that they are roaming even when they are not.
They then warn you in case it might cost more than when you are on the home network,
In this case it doesn't matter and doesn't cost anything extra.
If you look at your settings there may be one that can be enabled to eliminate the message.
See what roaming options there are (e.g. enable data roaming, enable national data roaming etc.) and see if any of them eliminate the message.
Or there may be an option in the messaging client to suppress this warning.
Otherwise you will just have to live with it and ignore it.
Hope this helps.
