I just googoo the number....it always comes up with a load of nuisance tracking services that feature that number and it's credentials (or lack thereof!)
If I accidentally answer one (most desist after 6 rings) i stay silent...friends know to initiate conversations, automatic spammers think it's an answerphone and drop the call.....in the case where a pre-recorded message starts, I put the phone down....after a few minutes,the "whee wah" noise alerts me to replace the handset. A live caller is asked "do you understand *telephone preference service?* remove this number from your dialling list NOW!
Always an apology and hang-up. Winds me up......refuse to answer any mobile call where I don't know the number.
Friend said to answer and string them along....I think their bulk minutes are cheap as chips, so it won't impact them.