This number just called me. After reading this post, I checked my call forwarding to find that it was set to send my calls to this number 215-920-9498.
This is your AT & T voicemail service's telephone number! You access it by holding down the #1 - I find I cannot call it from a landline. It will send you a message whenever a caller has left you a voicemail. You can hear the message by answering it - rathering than calling into the voicemail box. If you want to de-activate it, go to call forwarding settings and delete the number.