If Dylan used the landline on Monday, I think we would know about it. Clearly he did not contact R, since R was texting him throughout the day. Nor did ER hear from him, apparently. If anyone had, Dylan surely would have said his cell was not working, IMO. Also, if LE knew for sure Dylan was alive and well at any time on Monday, I doubt they would still be including Sunday night in their communications to the public.
If they thought, for instance, that a predator might have followed them on Sunday night, surely LE would be showing photos of MR's truck to the public, again and again. Yet have they shown it, even once,asking for any info about it?
Also, I find it so unlikely that Dylan would simply stop texting at 9:30 on a non-school night, when according to MR he was up for at least another hour, if not longer. Movie or no movie. They text all the time and he had just arrived in town, where he seems to have had many friends.
To me, at least, it is clear that LE has no proof that Dylan awoke on Monday.