Modem reverts to PSTN when set for leased line operation

These products can revert to PSTN settings when dial back-up / Leased Line restoral is enabled.
To disable Dial Backup, Leased line restoral:
Set Register's S15 and S17 to 0 (zero) and save the settings.

Example:
ATS15=0
ATS17=0
AT&W