Hi all we have a Vigor 2862Ln its all setup and working fine except...
when we cut over to the LTE connection which we use for FailOver our VOIP phones can make calls but we cant receive them the phone will ring then stop.

can anyone point me in the right direction please.

It's possible that your regular connection is routable / directly accessible via NAT but the LTE connection may not be. You can try setting up STUN or another NAT traversal method on your phones to see if that lets you receive calls.
If that helps, it should be fine to leave that enabled with either WAN connection.

If you have a proper 3G/4G backup (like e.g. Andrews & Arnold offer) then your fixed public IPs should fail over to the 3G/4G system, and it should carry on working as normal.

This is what I have at work and I was testing it yesterday.

So the NAT layer which a cellular network normally gives you is not relevant.

Inbound VOIP behind NAT cannot work. There are two approaches I know of: (a) some form of keep-alive activity from the phone which keeps the NAT channel open (most routers close it at 180 secs) and STUN is one approach which I have successfully used on VOIP on a phone or (b) port-forward the ISPs IP range and port 5060 to an internal IP where the phone is.

If your cellular backup is not getting IPs transferred to it by the ISP, inbound calls will never work because they don't know where to find you.

I must add that 3G is no good for VOIP. Very marginal quality and breaking up. One needs 4G. No idea why - probably the packet latency. Same with VOIP on a phone; you need 3G+HSPA as a minimum and preferably 4G. This should not be needed for voice and I have long suspected that the cellular networks have done this deliberately to avoid revenue loss by lots of people dumping GSM and using their huge data allowance all over the EU for making calls.