When setting up a new SIP trunk it's always best to test calls both inbound and outbound and to test outbound calls to more than one destination type as behind the scenes, calls can and are routed via more than one carrier in order to provide the best possible rates and to ensure resilience.
These carriers may accept different CODECs and so different 'transcoding' may apply via one route compared to another: if your calling PBX is advertising a CODEC that is not supported, may result in a call being dropped almost immediately, so if this issue takes place a typical outcome is that you may find that calls to one route (e.g. UK local / national calls) will function, whilst calls to another (e.g. UK mobiles) may not.
The solution if you experience this is to ensure that your PBX is not correctly configured with the accepted CODECs (please refer to 'What CODECs does SIP Trunk Plus support?' and, if you are configuring a 3CX system, please refer to ''How do I configure a 3CX PBX for SIP Trunk Plus?).