Grandstream's VoIP-Phone BT-101 allows up to 192 kBytes of custom ringtones, divided into a maximum of three 64 kByte blocks. As it is using the ULAW compression, one block can be up to 8 seconds long, which allows the following ringtone combination:

• 3 ringtones, each up 8 seconds long
• 1 ringtone with up to 8 sec, 1 ringtone with up to 16 sec
• 1 ringtone with up to 24 seconds

Generation
Grandstream supports two ringtone generation tools, one sox based for Linux and Solaris, and another one for Windows. I first tried the Windows tool, but you have to provide 16 bit, 8kHz mono WAV files, otherwise it fails with error. Additionally, sound quality was more than poor.
So I fell back to the Linux tool, for which I first had to install the distribution specific sox (apt-get install sox under Debian). Then I created a 16-bit, 44kHz stereo ringtone with 3,5 seconds, copied the file to the tool directory and typed the following command:


Upload to phone
At boottime the phone connects to the configured TFTP update server and looks for updates - including ringtones. The custom ringtones have to be named ring1.bin, ring2.bin and ring3.bin. It first tries to download ring1.bin and installs it if found. Depending on that ringtone's length, it will continue looking for tone 2 and 3.
I renamed ring.ring to ring1.bin and copied it into an empty directory (I could also use ring3.bin, but I wanted to overwrite the first ringtone, the other one's were alright). I configured my TFTP-Server Pumpkin to use that directory, started it, reconfigured the BT-101 to use my local PC as TFTP-Server and rebooted the phone. The ringtone was immediately installed and I could selected it using the phone's menu.