I have also had to acquire birth certificates from Nyasaland days.
I was nervous about paying over the phone to a foreign country, but the bank details I was given were proven to be correct and totally safe.
This is the information I used in 2009
The telephone number for the Registrar General’s Dept is
+265 1 824355. The person to talk to is a Mr. Tembo whose
Cell Number is +265 999 151377.
I hope this is of some help.
He is very helpful - good luck