Just had the exact same issue with alarm system after upgrade to new modem (upgrade probably 6 months ago). Called ADT first and they processed over the phone simple test and indicated there was an issue with phone line not getting a signal back to them. Called Comcast and they did the same simple test and said their system (phone, internet, and cable bundle) was working fine. Called back to ADT and they could only sent out a technician to run diagnostics (at a cost). The technician tested their complete system with their diagnostics for signal to the system and signal out or their system and all is working fine yet when the system tries to send a signal out via the phone line it does not come through. The technician also made comment that the previous call he was on had the same issue with signal not getting through on Comcast land line.
He did offer a wireless upgrade to the existing system or a complete new wireless upgrade – again at a cost.
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