Actually any broadband connection will work - the PBX will make use of whatever Internet connection you have.
The issues is more a matter of available bandwidth for placing VoIP calls. Because DSL is generally available at higher speeds and m
uch cheaper than traditional WAN technologies such as ISDN or leased lines, DSL is generally preferred. Without broadband access of any kind, however, calls will still have to be placed over the PSTN which effectively minimizes the potential cost savings on telecommunications.