My first guess would have been the filters - but as it's not that, it may be worth trying a few things to see if you can narrow down the problem.
If you unplug all of the phones (but keep your ADSL router/modem plugged in), try calling your landline from a mobile number and see what happens.
Did you make any other changes apart from the router? It seems that there may be a difference in tolerances with the router, or a problem with it.
I know little about how ADSL works, but I know we've got much more knowledgeable people on here - so hopefully you'll get some further ideas

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