Internal Calls Failing Digium P310

Afternoon, I have an issue as follows, and hope someone can help:

I have a site with 6 x D60 and 18 x P310 phones, these are connected to Freepbx held on a cloud server, I have setup multiple site previously but this one seems to giving me a slight problem. All the D60 and 14 of the P310 work as required, however I have 4 x P310 which can receive internal call, make external calls but cannot make internal calls.

All phones are registered with the PBX, and configured the same way, these 4 phones have nothing different configured.

The PBX is version: FreePBX 15.0.37.5

The phone are all on firmware 4_23_2

I have tried configuring the phone through adding to EPM pre setup and doing setup through the phone itself, if I take the phones off-site it shows the same issue for the first few hours and then seems to work normally.

I have checked the logs to see what I get when you dial internally, however nothing shows up in the log for the extension related to internal call, does show information for call in and external calls.

Just wondering if anyone has any ideas, for what I can do next, many thanks in anticipation.

Regards Les