A return meter helps for any RF problems but you should have that if you are already doing HSD. As for as actual phone, a volt meter, a toner that will work on twisted pair (almost any toners except for the pocket toners that use a barrel for coax only), and a buttset are the only two pieces of test equipment you need. There are more advanced phone meters, but for VOIP will never need them.
This is CABL.com posting #176715. Tiny Link: cabl.co/mT8p