In its operation instruction, the one of problems is stated as:
"This product is unable to make calls when:
– the handset batteries need recharging or have failed.
– there is a power failure."
Since the problem is not about the power, it may follow this condition:
"If you cannot make calls, change this setting
according to your telephone line service. The
default setting is “Tone”.
“Tone”: For tone dial service.
“Pulse”: For rotary/pulse dial service.
1 [MENU]#120
2 Select the desired setting.
3 [SAVE] and [OFF]"