No dialing sound / feedback to callers on incoming calls

Callers complain that they don’t hear anything after dialing our number (no ringing) until we pick up the call; accordingly they don’t know if their call is in progress or if our phone is even working as they don’t get any feedback until we actually answer the call or receive our VM message. These tend to be landline callers. When I call my Ooma number with my cell phone, I get normal feedback (periodic ringing).
Hardware setup: switched from a cable modem (Comcast) to DSL modem (Centurylink) several months ago. Using a single base phone with 4 wireless handsets. Ooma user since Nov-2010 and very happy with service otherwise.

please provide a call example (date/time/zone)