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
If you have some users whose extensions cannot be transformed by a pattern to the Caller ID value you want for them, you can set a specific value on the User Caller ID screen; as with the gateway, you can check the 'Block Caller ID' to make all calls made by the user anonymous:
Unless the gateway configuration has set the 'Ignore user Caller ID' flag, a user specific value takes precedence over the value set by a gateway.
Caller ID can also be defined at the User Group level. If set in the group and not overridden by the specific user, all users in the group share the same caller ID number for outgoing calls.
As with any group setting, if a user is in several groups, the setting of the last group is the one that controls the active caller id.
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.