If you are using the generic sip trunk option, there is a KNOWN bug in it that overwrites the CNAME...
If you export the file you will see the following:
<field name="ParameterIn" custom="" parameter="FromUserPart">$CallerName</field>
<field name="ParameterIn" custom="" parameter="FromUserPart">$CallerNum</field>
The developer copied and pasted but forgot to change the parameter name so the CNAME gets overwritten.
Export, change it to the below, then go to advanced, templates and import under the provider option.\
<field name="ParameterIn" custom="" parameter="FromUserPart">$CallerNum</field>
<field name="ParameterIn" custom="" parameter="FromDisplayName">$CallerName</field>