What frequency are your cordless phones? I had issues with 2.4Ghz phones in that past as that's what the router also runs on.
If it's a mix, what happens when you just unplug the 2.4 ones?
Next troubleshooting; have you tried unplugging different phones and seeing what happens. I know once with an odd POTS phone issue I ended up tracing the problem to call caller ID box.
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