The sipXecs proxy can manipulate what it sends to your connection to the PSTN, but different PSTN connections have different capabilities in terms of what is possible from there:
Getting all this to work so that the id presented by the PSTN is what you want it to be may take some experimentation and reconfiguration. See #What caller id is in SIP below.
Gateway level Caller ID
There are a few different ways of manipulating the caller id that is sent to a given gateway; each of these apply to any call sent out through that gateway. Normally, any of these settings may be overridden by settings that are specific to a particular user as described below; to prevent a user-specified value from overriding the gateway value, check the 'Ignore user Caller ID' checkbox.
Setting a single Caller ID for all outbound calls
Between that From header field sent by sipXecs and the caller id display on the phone of the person you are calling are any number of possible transformations. It may be that it will require reconfiguration of one or more of the elements of that signaling path to get the result you want, and it may be that some part of your connection will prevent what you want. It will probably be helpful to verify how sipXecs is setting the value; to trace this, see Display SIP message Message flow using Sipviewer.