If you've recently bought a pre-owned Hiptop (1, 2, 3 or 4) you'll have a world of trouble 'registering' it on the network. Simply put, they will have been registered by a previous user so will contain all of their information.
This information will need to be wiped in order for you to use the Hiptop.
So, if you want to use the Hiptop with Telstra and sign up on a 24-month plan ($30 per month) and use all of it's fancy applications (internet browings, unlimited SMS and MMS), you'll need to contact 1300 131 816. This is the number for Telstra's wireless data helpline and they really do help.
Be aware that you're NOT asking them to unlock your phone - pretty sure they won't do that for you - you're only asking them to clear the previous user's details on the network so you can use the phone to its full capacity.
Peripheral information is that because you've already got a phone, you can only sign up for a 24-month contract, not 12 months as I was previously told.
Hope this helps.