Calls to your mobile take longer to set up than calls to a desk phone.
For example, if you call a desk phone from another desk phone internally, the phone begins to ring as soon as you hit the last digit.
But when you call a mobile it may take several seconds for the mobile to ring.
There are several timers that may or may not be involved here depending on how the desk phone is call rerouted/forwarded and a couple of other options. The simplest thing to try is to change the call forward timer in the COS of the phone and extend it.
If that doesn't work then look at your call rerouting. If it's using a 2nd alternative option other than 1, change it to 1 and then at the phone forward it on internal/external No Answer to VM and try again.
Ralph