John:

As you haven't given a full postcode, we can't check your predicted or indeed if you might be vulnerable to from a new/upgraded phone .
Have you received a postcard from Restore TV?

was on Planned in early August, but isn't currently listed, but we know sometimes the lists aren't 100% accurate.
As you mention your is high and amplified, it suggests you aren't in a particularly good reception area, but without the postcode we are unable to comment further on that.
There's also been weather conditions at times during recent months that have resulted in Tropospheric Ducting where interference from distant transmitters can occur or indeed just disrupt reception of weaker signals.
If the aerial system is 20 years old then, there could be degradation at the aerial connections, or to the downlead etc. If water has penetrated then this will cause issues as well.

Where is your amplification? If it's in the then check it still has and look at the coax connector on the end of the lead for the aerial and check for water or corrosion.
Check your aerial is still pointing correctly, again without the postcode, we can't give you the direction/compass bearing, but it's rods (or squashed Xs) should be horizontal.

We could probably advise further checks if we have a bit more detail about the aerial installation, amplification etc.

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