Check in the manual.
Is this your one?
http://documents.knowhow.com/Audio-Mobile%20Phone-Satnav/L15_25_35DTAM10_IB_A6_final100804_running%20change2.pdf
Based on your description, it might be the handset is telling you it is unable to communicate with the base station.
Is the base turned on and in range?
saafda, March 2018