The DSC 1555 DTMF levels are barely 0.4v p-p. When the alarm dials, the line is picked up, but the Linx does not detect them and emits the dial tone continuously.
While the DSC 1555 levels are lower than the phones, they are still within DTMF spec.
It does not matter which mode the Linx is in telephone, alarm or FAX.
As an aside, Ooma does not engender much good will when support states "We don't support alarms" I'd have never bought the thing if the Linx page hadn't stated "The Ooma Linx allows you to wirelessly connect additional telephones, fax machines or home alarms to your Ooma Telo"