Digital mobile phones always send keypress events in the signalling channel, not as audio tones. In most cases that is also true of SIP. You therefore have a rather complex system as far as how the 1 would get back to the voicemail application.
My suspicion is that it is being lost before Asterisk, and therefore you need to contact the OpenBTS people, as it is very unlikely that anyone on this forum has experience of it. However, it could be a misconfiguration between OpenBTS and Asterisk as to how DTMF digits are handled, or even a problem in Asterisk.
You really need to get detailed tracing information from both OpenBTS and Asterisk to see how far the keypress signal is getting. I can’t imagine that OpenBTS doesn’t support detailed tracing of both the air interface and the SIP side.
I’m somewhat concerned that you do not have strong contacts with the OpenBTS people, as my impression is that this is not the sort of system that could be legally used by a hobbyist or a non-specialist business.