IIUC, FreeSWITCH does not generate the dialtone. The VoIP phone will do that itself, and an ATA will generate it for a analog (POTS) phone. Your message wasn't clear if you are using VoIP phones, or an ATA (analog telephone adapter) with analog phones. A VoIP phone will "subscribe" with FreeSWITCH to a MWI login, and FreeSWITCH will send it a message whenever the number of messages waiting changes. In response, the VoIP phone will light the MWI and will play a stuttering dialtone when the handset is picked up.