There's no guaranteed you'll get a text..
The panel's modem dials an SMS gateway on a fixed telephone I guess your not going to pay rental on exclusive line for the alarm ?
So Ms Smith leaving an answer phone message, an incoming fax, a handset left of hook a line fault/ line cut by burglar will prevent an outgoing call
SMS gateways don't generate an income for the network should it stop working their a low priority to get repaired, some networks no longer support there's,
In times of high demand texts can be delayed or lost.
Its not really suitable for alarm signalling,
If you really wants texts there's SMS-Com thing which takes a sim card which is better but again its not alarm signalling