Who called from 3605240201

This number or variation of the last four keep coming up on my son's cell phone at all hours. Who is it?

I received a call from this number and they left an automated voicemail.  With a callback number (877) 340-2028 and a reference number (my cell phone number).  They said it was a collection agency, but that doesn't make sense.

Keep getting repeated phone calls from this number and several other variations. They mumble unintelligable things when I pick up and never leave messages. Anyone know how to make them stop calling? I have asked repeatedly

I called it back and it said that it was Verizon Wireless message retrieval center.