Welltech WG2504 FXS V.4.02 - Manual (Page 79)

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At this example, they are number is 2001.
c. FXO TEL NO. : See Figure 10 to configure TEL NO from FXO Setting
 FXO Line Modify Line Setting  TEL NO. Here, have four numbers
from 1001 to 1004. This option allows you to configure your desire Line
number to deliver to remote FXS gateway.
See figure 14. FXO Gateway Caller ID Mode.
Figure 15. FXO Telephone Number.