make sure the handset is in group & not on dnd or forward unconditional.
this is the cause of the problem you are reporting in almost all cases.
sometimes the ringer goes silent, changing the ringer volume usually resolves this.
Amridle the DT/20xx handsets came in 4 variants
DT1/2010, basic phone no display
DT3/2030, Display phone with hands free no BLF lamps
DT4/2040, Display phone no Handsfree, BLF lamps
DT5/2050, Display phone with Hnadsfree & BLF lamps
(& to be totally accurate there was also the short lived DT6
which was a DT% with RS232 to give Basic CTI on an INDeX)
DT3/2030 & DT5/2050 wer by far the most popular versions