I very often get spam robocalls to my cell phone (on T-Mobile US prepaid) that come from a number identical to my phone number but with the last 4 digits altered (and random every time).
FYI, T-Mobile US rolled out a free spam blocking feature at the start of the year, but you have to opt into it (presumably because of the risk of dropping valid calls). I don't know if it applies to prepaid, unfortunately. If you log into your account it should be listed under features you can add to your line.