Make sure you also check to that Personal Hotspot is enabled on the network/service if you want to use that. It's often an additional fee to make it available.
It was also confusing trying to work out whether I could send and/or receive International SMS or MMS from the US number. The website seemed to indicate that I had to have an additional "International" pack attached (which I didn't), but then I could still send SMS that sometimes seemed to get through to my family back in Oz and sometimes didn't. Same for receiving: seemed to be irregular.
And I had my Aussie mobile diverted to my Skype online number so people could leave a message that I could retrieve from my Skype voicemail if they rang my mobile (seeing I didn't have my Aussie SIM in my iPhone).
https://help.telstra.com/app/answers/detail/a_id/17332/c/1986/kw/divert/r_id/130958 (I'm assuming you're Telstra). But when I got back, it took two phone calls to get it switched back to divert to my normal Telstra message bank. (From experience, don't do this after 24 hours flying! Your patience wears a little thing.)
The only downside in all this was I couldn't receive any text messages that people sent to my aussie mobile unless I swapped the SIM in and out, but this meant I had to re-enter the T-Mobile data APN settings each time etc. Does anyone know a way to divert SMS messages?