Network keeps getting hacked
So ive been working for my unc in his office lately and we keep getting an issue where people keep hacking into the network. I dont know what it is but people keep hacking into the voip phone and we can hear their conversations. When we phone up the voip guys (viva communications) they reset the factory settings and it stays ok for like a week but then it gets hacked again. The router is a technicolor tg582n so its not that good. I need to improve security.
I changed the wifi pass (WPA2-PSK + WPA-PSK) and the router admin pass. are there any other things I can do? Are there any good routers I can get for preferably <£40?
Casual gamer and likes programming and stuff.
Asus Zenbook Pro with a GTX 1050