Dear brother how you checked it on other lines because you have to change the DSL username and password inside the modem settings for that particular line/number. If you can access the modem settings then you can check it's speed limit in WAN settings, may be it is set to 256kbps
(type //192.168.1.1 in the address bar and enter "admin" as username and password to access modem settings).
And with 256kbps you can only acheive 32 KBps speed max, it is logical(256kbps/8bits = 32kBps).
If there is no issue with the modem settings then your line must be a victim of distortion, so also verify the SNR and attenuation level of your line.
If your line and may be all the near by customers attached with the same DP have distortion issue then the PTCL limit the speed themselves to avoid disconnection.
Please also confirm the level of noise when you hear a call.
Regards