If a VoIP line that is in a ring group has been forwarded to another number (other than the ring group number) such as a mobile number or landline then this will affect the ring group of that line - meaning that the phones in the ring group will not ring as the line has been diverted.
If the line has not been diverted please check to see that the line is not off line by checking the line summary status please click here to see further information on this.