T20P stutter duration too short

[AndreRossiter]

Stuttered dialtone has been configured when there are new voice messages but it only plays for a fraction of a second whereas I would like multiple seconds (my clients are blind so visual indication is of no use).

I have experimented with the Web UI Custom value for Stutter and have successfully changed the initial tone but never the duration. Following multiple examples, I confirmed my syntax was good but still the duration does not take effect.

Some samples I tried:

350+440/1000
350+440/5000
!350+440/100,!0/100,!350+440/100,!0/100 (default value in Asterisk for US)
480+540/2000 (strange tone that confirms the change aside from the duration)

I also changed the country value and again the duration is minimal.

Rebooting the phone made no difference.

Other info about the phone:
Firmware: 9.70.0.137
Hardware: 7.0.1.62