Keep in mind that sometimes the cheapest phones work the best with VOIP... The VOIP protocol does compression and correction on the conversion of your voice. Some of the higher end phones do the same thing so with VOIP sometimes you get weird things happening with the double correction. More expensive/higher quality phone doesn't always mean better with voip..
Interference from sources in your house is one thing, but nothing compared to the problems you can get with a bad phone to viop paring... Same voip, same internet connection, same network, 2 different phones can be a world of difference.
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BellieLive