This is caused by an incompatible DTMF mode configured on your phone.
Voice servers try to detect what DTMF modes your phone is able to support, but this detection fails with some models. In particular, Siemens handsets seem to be problematic.
To work around this issue set the DTMF mode on your phone manually to be RFC2833. You'll find the DTMF mode in the connection settings for your handset. The precise name of the setting varies manufacturer to manufacturer. (Most voice servers use the RFC2833 protocol however check with your VoIP provider if you're not sure)
As this is quite a common issue with the Siemens Gigaset models, please find the step-by-step instructions below as to how to change this setting:
Firstly, you will need to find out the IP address for your phone. To do this:
- Press the Menu button on your handset
- Go to following menus - Settings > Base > Local Network and enter your system pin*
- Scroll to "IP Address" and make a note of the number on screen. This will be presented in a similar way to "192.168.001.063". In each portion of this number, remove any leading zeros so, in the example above, the IP address will be 192.168.1.63.
- Open your regular web browser (Internet Explorer / Firefox / Safari) and, delete EVERYTHING in the address bar and, type the IP address you discovered previously, and press enter.
- On the Gigaset login screen, enter your system pin*
- From the top menu, click Settings
- From the left menu, click Telephony, then Advanced Settings
- Under the area "DTMF over VoIP", take the tick out of all three options, then put a tick in "RFC 2833"
- Click the Set button
After changing this setting, Click the Logout option on the Gigaset Web Configurator.
After making this one-time change, your phone will now operate correctly with the voicemail system (as well as any other phone numbers that require you to press numbered options).