The volume on my Panasonic cordless phone is variable?

Model kx-tg5922al (5.8ghz)
I have the volume set on full the total time, but often we have to strain to hear the other person. Not always even if – sometimes clear as a bell. Yes we have wireless internet which I thought at first might be interfering, but it doesn’t seem to be related. Maybe its the Telstra line (Australia). I have heard others say they have similar problems with their cordless phones.
I have tried a corded buzz on the same line and there was no problem, but I reflect cordless phones might be more prone to drop in volume when the landline signal is weak. I would like to hear from anyone who has had similar problems. It would confirm my theory at least. Then I can contact the communications ombudsman to see what Telstra will do about it .

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