One major point to note, especially with 3Com models is that they have to be within the allowed MAC ID range for them to be accepted on the network. A drawback with this is that if you had bought your modem (via Global etc.) and it breaks and you have a replacement under warranty it may not work
3Com have shipped replacements that are NOT in the allowed range in the past. The only option here is to have an ntl 'approved' modem shipped to you from customer services.