Well this is going to be an easy a short post.
I am quite convinced, there are quite a few people around that have analog phones deployed in a, for the rest, all out IP phone deployment. I have seen this a number of times, analog phones on FXS ports (MGCP, SCCP or H323) and I have also seen issues with transfers on these phones, putting callers on hold and consultative transfers.
Some times these issues can be resolved by tweaking the minimum and maximum hook flash timers on the voice-port to which the analog phone is connected (even when the FXS port is MGCP controlled).
An alternative to consultive transfer is to configure directed call park.
The CUCM Features and services guide 8.0(2), defines directed call park as:
Directed Call Park allows a user to transfer a call to an available user-selected directed call park number. Configure directed call park numbers in the Cisco Unified Communications Manager Directed Call Park Configuration window. Configured directed call park numbers exist clusterwide. You can configure phones that support the directed call park Busy Lamp Field (BLF) to monitor the busy/idle status of specific directed call park numbers. Users can also use the BLF to speed dial a directed call park number.
A user can retrieve a parked call by dialing a configured retrieval prefix followed by the directed call park number where the call is parked.
So in the case of an analog phone wanting to do a consultive transfer, the called party transfers the caller to a directed call park number (for instance *12345). this will put the caller on hold. The called party now dials the party the transfer needs to go to and informs that party that a call is parked on *12345. The end party can now retrieve the parked call and the original caller is taken of hold.
This whole process is transparent to the original call as the experience will be that that person has been put on hold.
I'm just throwing it out there, good luck with it.