Hi z1ts,
Although you might only have a very few real alarms, the unit should have a watchdog type of service with regular polls, possibly through either or both comms channels.
I would expect that the phone line is the normal route and the mobile is only a standby one.
As the PSTN connection is likely to be a dial-up modem, it might be prudent to keep those signals well clear of VDSL services.
When your VDSL service fails, you really don't want to be poking around with those wires if they were involved with the alarm system either.
Kind regards,
Walter