As a guess, try setting for your trunk From User to the same value you have in Username and From Domain to the same value you have in SIP Server.
If no luck, please post all the settings in the mobile client and we can give you the equivalent for FreePBX. Please don’t use “x”; use these names to represent values where applicable:
myusername
myaccountnumber
mypassword
myphonenumber
mydisplayname
mye164phonenumber
providerdomainname
provideroutboundproxydomain
provideroutboundproxyaddress
providerportnumber
If there are other parameters required by the provider, choose a suitable name so we understand what it represents.