Setting up Andrews and Arnold VOIP with a Grandstream HT802 v2 and Plusnet Hub One

[#1]

Perhaps leave that voltage unchanged because ....

If the limit is 55v that field is more like the constant line operating voltage ( 48 - 50v DC is the norm )

The Voltage for ringing the bell of the phone at the start of a new call is usually much higher 60 or 70 up to  85 or 90v AC.

[#2]

A&A have test numbers:  https://support.aa.net.uk/VoIP_-_Special_Codes

Even calling to the A&A Voice Message service would allow making repeated calls
to test the stability of the in-bound voice stream.