[chopsim]

We have renovated a new office which has foil backed plasterboard so the phone signal is awful.
The computer internet connection is hardwired into the main router in the main office.
What's the best booster for that? There is no WiFi coverage in the new office either.

[moox]

How big is the office?

If it's a small business, and you are all using the same network, you could get a femtocell / signal box from the network you use. EE, 3 and Vodafone all offer them (EE/Vodafone will just sell you one, 3 will make you jump through a couple of hoops but it is free). O2 might give you one if you pester them.

Your chances of getting one will be increased if you've got some company mobile plan

These require a hard wired connection to the router.

If you have modern phones and you can install WiFi, then you could also just use WiFi calling.

[chopsim]

We have a mobile phone plan with Vodafone with over 70 phones on it. The office phones are all IP phones. The main office has the server etc in. His office which is about 30 yards away on the other side of the yard has an IP phone hardwired into the router in the main office. He can't get WiFi in his office.

[moox]

I'd look at getting Vodafone involved, they may be willing to give you a free Sure Signal box at the very least, and maybe even something a bit beefier since you have a lot of company phones.