How do I achieve this?

Settings > Mobile Data > Mobile Data Options > Enable 4G for Voice and Data

I work next to a 4G mAst but when I call my phone uses 2g why?

Is it on 4G before and after a call ?

You need to be on a contract, not PAYG. VoLTE (4G voice) doesn't currently work for PAYG customers. It will one day.
If you don't have access to VoLTE for the above reason then your iPhone will switch down to 2G or 3G to do calls.

I have a iPhone 6s on contract and where I work there are 2G and 4G on the roof of the building, my phone sits very happy with 4G but when I make a call that disappears as the phone goes on 2g

Go to settings - general - -about and press on carrier. Does it do anything?

Have you followed instructions above?

Mate if you have done everything and your phone is voice and data on 4G settings and contract phone then I think it is time to call EE may be?

Do all EE masts/areas now have VoLTE ? I think Mr Satman lives in Ipswich ?

Yes. VoLTE rollout was completed in July I think.

You didn't say whether you'd followed Mark's instructions and turned on Voice & Data in Mobile data settings. Have you?
If it's set to Data Only then you will fall back to 2G/3G for calls.

It's on calls and data yes

Have you followed my instructions?

Yes all instructions have been following

dont waste your time. just speak to EE. They may be able to find out for you why you are not getting 4G Calling in 4G area with compatable VOLTE phone on monthly contract. Thay wil run diagnostics on your phone and sort it out as they did with my as my sim was not updated