Some of our customers are assigned phone numbers that were previously assigned to somebody else. We do everything we can to ensure that a “new” number is given to our customers, but from time to time these numbers are still in people’s old address books.
Our first recommendation would be to block the number, most devices will include a native call blocking feature.
iPhone
If you're in the Phone app under Recents, tap 
next to the phone number or contact that you want to block. Scroll to the bottom of your screen, then tap Block this Caller.
Android
In the Phone app you will just need to long-press the number, and select Block or Reject Caller.
There are also third-party applications which you can install that blocks and manages calls for you – try iBlacklist for iOS or Call Control for Android.
Unfortunately there’s not much we can do to block unwanted calls at the network level. If you’re receiving telemarketing calls, you can register your Roam number with the National Do Not Call Registry.