[SOLVED] Called extension does not ring - but BLF lights up

FreePBX-Distro: 10.13.66-19
Asterisk Version: 13.14.0
FreePBX Version:
PBX Firmware: 10.13.66-19

We use SNOM870 phones all running Firmware-Version snom870-SIP We use the Commercial Endpoint Manager to configure them. Each phone is set to update its settings every 24 hours.

I have a problem with one user/device-extension that does not ring when called. It does not even ring when the web interface for the phone is used to test the ringer. The BLF for the extension flashes but the phone does not ring.

I have compared the setting of this phone with those of another that does ring when called and see these differences:

<pnp_config perm="">off</pnp_config>
<network_id_port perm="RW">5060</network_id_port>
<custom_melody_url perm=""></custom_melody_url>
<firmware_status perm="R">http://3402a049:[email protected]:83/snom870-000413419A8A-firmware.xml</firmware_status>

Does not Ring:
<pnp_config perm="">on</pnp_config>
<network_id_port perm="RW">5061</network_id_port>
<custom_melody_url perm=""></custom_melody_url>
<firmware_status perm="R">http://3402a049:[email protected]:83/snom870-000413419BD1-firmware.xml</firmware_status>

CEPM Basefile template
<network_id_port perm="RW">__line1sipPort__</network_id_port>
<firmware_status perm="R">__firmware__</firmware_status>

Since all the other phones do not exhibit this problem; and given that the extension settings displayed in FreePBX are identical for all these units; I am working from the premise that it must be a local setting that is causing this issue. However, I cannot locate which setting specifically deals with the ringer volume.

I have since altered the device SIP port setting to 5060 and this seems to have changed the behaviour.