Hello Maria ,
The Internal calls are ok? or this is happening with outgoing calls to the puplic provider?
Also Is the base station 4070 installed to the expansion cabinet? Try to set ibs cirquit to the main cabinet and see if the behaviour is the same.
If this doesnt work or is in the main cabinet set diferent ua card/port. Also i installed 8232 to 710_94 with success maybe to downgrade it ?(but i dont think this is the problem)
also disable eco mode for transmition power maybe you are away from the base station and the signal is weak??...