I have connected an M700 to a Grandstream UCM 6304A in order for my customer to take advantage of the M80 handsets for use outdoors.
I have set up the system on my bench and tested everything. I have everything working the way that I wanted. One of the features that the customer most needed was the ability to park calls.
Grandstream allows this using the #72 Feature Code.
I happened to notice just before packing everything up to take to the customer that I had forgotten to update the firmware on the M700, M70, and M80’s.
The M700 was running v4.10 when I got it. I cannot recall what versions the cordless handsets were running prior to updating them.
I have the M700 updated now to 520.2 and all of the handsets running 650.2. Now that I have performed the firmware updates the cordless handsets are no longer able to use #72 to park a call consistently. Sometimes it works without issue, other times I have to enter #72 two or three times before it works. 

Has anyone encountered any issues with sending touch tones to an onsite PBX following the firmware updates? I'm assuming that this would be related to tones. I don't really have any better ideas for why this isn't working consistently.
I tested this several times before updating the handsets to 650.2 and hadn't noticed any issues with initiating a call park via feature code.