This is not a TPG issue, or a TP-Link issue. If you bridge the TP-Link so the D7000 becomes your internet-facing router, then any functionality which the TP-Link had is no longer used because it's not doing any routing, just being a modem for the VDSL. The problem is that your D7000 doesn't have VOIP. You either use the TP-Link and port forward there, utilizing your D7000 as an access point, or you do away with both and get an all in one like a Draytek 2862VAC or perhaps an Asus AC2400 - the Asus is pretty cool because it's also a DECT phone base station built-in so you don't need to connect any phone lines other than your VDSL telephone line.
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