I'm guessing that on your SIP Trunk-Group the Service Type is set to Public-Network. Try changing it to Tie.
CM assumes that a Public SIP Trunk-group is connecting to the PSTN through a SIP Service Provider. In such cases, the SIP Serviced Provider requires that the Called-number be formatted as E.164. And of course, E.164 numbers are identified by the leading '+' sign. So, CM automatically adds the '+' sign.
CM assumes that a TIE trunk is connecting to other servers within your environment. In such case, the private numbering scheme could be whatever you need it to be. In that case, CM does not assume it would be an E.16 number, and consequently does not add the leading plus sign.