You can change the PSTN mapping to another number as follows:
1. Enter your account at http://www.didww.com/ and go to My DIDs page (Figure 1).

Figure 1: Access My DIDs Page in your Account
2. Select the DID number that you wish to change in PSTN mapping and click on PSTN Services of that DID (Figure 2).

Figure 2: Access PSTN Services Page in your Account
3. Erase the Phone Number that you wish to change, put the new number and press Update for saving your changes (Figure 3).

Figure 3: Change the PSTN Number
Please note that the country should be of the same Zone as your current one.In case this country is included in another Zone, another PSTN plan should be ordered.
Remember, the number must be entered in country-code+city-code+number format. The countries to which call forwarding to PSTN is supported, are listed in the Mapping page under PSTN.
* Please note that call forwarding to PSTN flat rate plan is for personal use only.
KB » PSTN (Land Line and Cellphone)

Can I Change The Number in PSTN Mapping to Another PSTN Number?