Free caller ID

My android detects this as spam and does not ring to my handset. Voicemails are some times up to 4 times per day. For the past 3 weeks, voicemails are 2 seconds long; empty - not even static. Prior to that, the calls were also suspected spam and did not ring through to my handset. The voicemails however were 3 seconds long, and consisted of nothing more than 2 tones (about 2 seconds of silence, then C# followed by F#, each tone lasting about 250 to 300ms each, with no pause between them. At the