equipment purchase help

hello, i am a new xfinity customer. i subscibe to 25mbps internet and home phone service. 
i wanted to buy a modem to use with these services. i did an online chat with a rep to confirm after utilizing these two links to pick out one to buy. 
https://www.xfinity.com/support/devices/#auth
https://forums.xfinity.com/t5/Your-Home-Network/Approved-Xfinity-Modems-and-Gateways/td-p/3109887

i purchased the Arris TG862G brand new from amazon. 
the tech installed our service but the voice wouldnt work. 

the next three days i did online chat with two different reps. the second said they were going to escilate my issue to a higher up tech and they would call back. 

i never got a call back. 

i finally contacted the gentleman who did the initial install. 

he came today and swaped my new modem with a rental unit and the voice is now working. 

i plan to return the unit i bought. the tech said it was a faulty unit, but im concerned...

should i buy a different modem? i dont want to order a replacement for the same model if its not going to be compatable with my services. 

the tg862g was listed as a unit that would work. the rep i spoke with confirmed it would work, but it hasnt. maybe it is just a faulty unit straight out of the package, or maybe i should buy a different model all together. 

the rental unit is a Arris TG 1682G.

i live in a small two bedroom home. i only plan to keep this 25mbps speed. i dont really need the latest and greatest modem with a huge wifi coverage area and support for 1g speeds. 

can anyone suggest me a unit that wont break the bank and will work? 

thanks for the help!!!