Solution : https://service.sap.com/sap/support/notes/591440 (SAP Service marketplace login required)
Key words : 
line number transferred, external line number, line number, external service, purchase order, system adds, service lines, customizing settings, esll-extrow, bapi solution
Related Notes : 
       
| 628397 | SAPKU30015: Support Package 15 for EBP 3.0/CRM 3.0 | 
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| 609353 | SAPKU30014: Support Package 14 for EBP 3.0/CRM 3.0 | 
| 597764 | Support Package 06, notes for EBP 3.5 | 
| 575041 | SP05 notes for EBP 3.5 | 
| 574952 | SP04 / SR 1 - Notes for EBP 3.5 |