A +++ doesn't escape the modem to hang it up

When using an external PPP stack, the +++ doesn't put the modem into online command mode.

Options for terminating a packet data call:

Send LCP Terminate request to the network (most ppp implementations should do this when they exit). This will cause the call to drop and the modem to return to command mode.

Set &D1 or &D2 and use the DTR signal to return to command mode.